Split out OCI store

[jedevc]

⬆️ Follow-up to #2827 #3118
⚠️ Breaks API compatibility with previous OCI implementation (should be alright, as long as it merges before v0.11 is released)

This PR performs a couple of OCI refactors on-top of the ones already done in #3300:

• Removes unnecessary caps - the feature only requires a single extra cap, instead of the 3 that were previously defined. Since the existence of the main OCI cap implied the existence of the other ones, we don't need to have the other ones, more caps should only be required as we increase the scope of the feature over time.

• Removes the unnecessary SessionID attachment to the normal docker image resolver, since it's only relevant when loading from OCI stores in the OCI image resolver.

• Refactors the store id out of the main image reference, and instead attaches the store id as a separate property. This prevent us from needing to do name mangling and de-mangling (preventing us from needing the Hostname call).

• Finally, since the reference then becomes completely irrelevant except for the digest portion, we can name it appropriately to ensure that we get reasonable error messages. Instead of using the store name as the basis for the dummy ref, we keep the original name, which allows the client to perform content store mappings as it desires - this pairs well with build: refactor reference parsing for oci image layouts docker/buildx#1456, which uses randomly generated identifiers as the store names.

  Before:

  $ buildctl build ... --opt context:x/y=oci-layout://dump@sha256:36e80b557af40b63856575bab09a71739026d1f8e3d31351e6852c06a0e5306a --oci-layout dump=./store
   => [context x/y] load metadata for dump@sha256:36e80b557af40b63856575bab09a71739026d1f8e3d31351e6852c06a0e5306a                                  0.0s
   => [context x/y] OCI load from client                                                                                                            0.0s
   => => resolve foo/bar@sha256:36e80b557af40b63856575bab09a71739026d1f8e3d31351e6852c06a0e5306a                                                    0.0s
   => => sha256:d93dab677ee66a618cb0662f39b2dff30ca7f89fe9beddb9aaad9a17b766fee5 7.00MB / 7.00MB                                                    0.1s
   => => extracting sha256:d93dab677ee66a618cb0662f39b2dff30ca7f89fe9beddb9aaad9a17b766fee5                                                         0.1s
  

  After:

  $ buildctl build ... --opt context:x/y=oci-layout://dump@sha256:36e80b557af40b63856575bab09a71739026d1f8e3d31351e6852c06a0e5306a --oci-layout dump=./store
   => [context x/y] load metadata for docker.io/x/y@sha256:36e80b557af40b63856575bab09a71739026d1f8e3d31351e6852c06a0e5306a                         0.1s
   => [context x/y] OCI load from client                                                                                                            0.3s
   => => resolve docker.io/x/y@sha256:36e80b557af40b63856575bab09a71739026d1f8e3d31351e6852c06a0e5306a                                              0.0s
   => => sha256:d93dab677ee66a618cb0662f39b2dff30ca7f89fe9beddb9aaad9a17b766fee5 7.00MB / 7.00MB                                                    0.1s
   => => extracting sha256:d93dab677ee66a618cb0662f39b2dff30ca7f89fe9beddb9aaad9a17b766fee5                                                         0.1s
  

[tonistiigi]

@jedevc Needs rebase.