Add new "bashbrew remote arches" command

[tianon]

This command will, given a remote image reference, look up the list of platforms from it and match them to supported bashbrew architectures (providing content descriptors for each).

(This builds on both #54 and #55 because the former creates the initial version of the interfaces this uses and the latter touches go.mod significantly enough to be worth not trying to do independently.)

I'd be totally happy ditching #54 in favor of this (and can roll up/squash that code), if you prefer, @yosifkit. 🙇

$ bashbrew remote arches hello-world
docker.io/library/hello-world:latest -> sha256:faa03e786c97f07ef34423fccceeec2398ec8a5759259f94d99078f264e9d7af
  amd64 -> sha256:f54a58bc1aac5ea1a25d796ae155dc228b3f0e11d046ae276b39c4bf2f13d8c4
  arm32v5 -> sha256:7b8b7289d0536a08eabdf71c20246e23f7116641db7e1d278592236ea4dcb30c
  arm32v7 -> sha256:f130bd2d67e6e9280ac6d0a6c83857bfaf70234e8ef4236876eccfbd30973b1c
  arm64v8 -> sha256:432f982638b3aefab73cc58ab28f5c16e96fdb504e8c134fc58dff4bae8bf338
  i386 -> sha256:995efde2e81b21d1ea7066aa77a59298a62a9e9fbb4b77f36c189774ec9b1089
  mips64le -> sha256:eb11b1a194ff8e236a01eff392c4e1296a53b0fb4780d8b0382f7996a15d5392
  ppc64le -> sha256:b836bb24a270b9cc935962d8228517fde0f16990e88893d935efcb1b14c0017a
  riscv64 -> sha256:98c9722322be649df94780d3fbe594fce7996234b259f27eac9428b84050c849
  s390x -> sha256:c7b6944911848ce39b44ed660d95fb54d69bbd531de724c7ce6fc9f743c0b861
  windows-amd64 -> sha256:fb353688bcf45fc724fde3d1dcd7935ddf56803e2b7027164a7acc28758002f6
  windows-amd64 -> sha256:c597fadc51747a7392f4d0312e9740aaee11793a47658a77a4ba51bd5d9b6be0
$ bashbrew remote arches --json hello-world | jq
{
  "ref": "docker.io/library/hello-world:latest",
  "desc": {
    "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
    "digest": "sha256:faa03e786c97f07ef34423fccceeec2398ec8a5759259f94d99078f264e9d7af",
    "size": 2561
  },
  "arches": {
    "amd64": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:f54a58bc1aac5ea1a25d796ae155dc228b3f0e11d046ae276b39c4bf2f13d8c4",
        "size": 525,
        "platform": {
          "architecture": "amd64",
          "os": "linux"
        }
      }
    ],
    "arm32v5": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:7b8b7289d0536a08eabdf71c20246e23f7116641db7e1d278592236ea4dcb30c",
        "size": 525,
        "platform": {
          "architecture": "arm",
          "os": "linux",
          "variant": "v5"
        }
      }
    ],
    "arm32v7": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:f130bd2d67e6e9280ac6d0a6c83857bfaf70234e8ef4236876eccfbd30973b1c",
        "size": 525,
        "platform": {
          "architecture": "arm",
          "os": "linux",
          "variant": "v7"
        }
      }
    ],
    "arm64v8": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:432f982638b3aefab73cc58ab28f5c16e96fdb504e8c134fc58dff4bae8bf338",
        "size": 525,
        "platform": {
          "architecture": "arm64",
          "os": "linux",
          "variant": "v8"
        }
      }
    ],
    "i386": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:995efde2e81b21d1ea7066aa77a59298a62a9e9fbb4b77f36c189774ec9b1089",
        "size": 525,
        "platform": {
          "architecture": "386",
          "os": "linux"
        }
      }
    ],
    "mips64le": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:eb11b1a194ff8e236a01eff392c4e1296a53b0fb4780d8b0382f7996a15d5392",
        "size": 525,
        "platform": {
          "architecture": "mips64le",
          "os": "linux"
        }
      }
    ],
    "ppc64le": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:b836bb24a270b9cc935962d8228517fde0f16990e88893d935efcb1b14c0017a",
        "size": 525,
        "platform": {
          "architecture": "ppc64le",
          "os": "linux"
        }
      }
    ],
    "riscv64": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:98c9722322be649df94780d3fbe594fce7996234b259f27eac9428b84050c849",
        "size": 525,
        "platform": {
          "architecture": "riscv64",
          "os": "linux"
        }
      }
    ],
    "s390x": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:c7b6944911848ce39b44ed660d95fb54d69bbd531de724c7ce6fc9f743c0b861",
        "size": 525,
        "platform": {
          "architecture": "s390x",
          "os": "linux"
        }
      }
    ],
    "windows-amd64": [
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:fb353688bcf45fc724fde3d1dcd7935ddf56803e2b7027164a7acc28758002f6",
        "size": 946,
        "platform": {
          "architecture": "amd64",
          "os": "windows",
          "os.version": "10.0.20348.1249"
        }
      },
      {
        "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
        "digest": "sha256:c597fadc51747a7392f4d0312e9740aaee11793a47658a77a4ba51bd5d9b6be0",
        "size": 946,
        "platform": {
          "architecture": "amd64",
          "os": "windows",
          "os.version": "10.0.17763.3650"
        }
      }
    ]
  }
}

[tianon]

(I added some basic godoc comments to the registry/ package in this PR, FWIW 👀)

[codecov-commenter]

Codecov Report

Merging #56 (b05f0ed) into master (8281cbe) will increase coverage by 1.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   76.15%   77.38%   +1.22%     
==========================================
  Files           6        6              
  Lines         604      619      +15     
==========================================
+ Hits          460      479      +19     
+ Misses        111      108       -3     
+ Partials       33       32       -1     

Impacted Files	Coverage Δ
architecture/oci-platform.go	100.00% <100.00%> (ø)
manifest/fetch.go	70.00% <100.00%> (+11.30%)	⬆️

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