buildctl (du|prune|debug info): support --format {{json .}}

[AkihiroSuda]

• buildctl du --format '{{json .}}': the template is applied to []*UsageInfo
• buildctl prune --format '{{json .}}': the template is applied to UsageInfo
• buildctl debug info --format '{{json .}}': the template is applied to *Info

$ buildctl du --format '{{json .}}'
[{"id":"s1d9pk3439ap6e5tt67bndctk","mutable":false,"inUse":false,"size":8979670,"createdAt":"2022-07-31T06:57:11.046458864Z","lastUsedAt":"2022-07-31T06:57:13.862022729Z","usageCount":1,"parents":null,"description":"pulled from docker.io/library/alpine@sha256:7580ece7963bfa863801466c0a488f11c86f85d9988051a9f9c68cb27f6b7872","recordType":"regular","shared":false},{"id":"u0nmub83lksjb9qu04y23v48m","mutable":true,"inUse":false,"size":2805760,"createdAt":"2022-07-31T06:57:11.68658936Z","lastUsedAt":"2022-07-31T06:57:13.858709691Z","usageCount":1,"parents":["s1d9pk3439ap6e5tt67bndctk"],"description":"mount / from exec /bin/sh -c apk add --no-cache neofetch","recordType":"regular","shared":false},{"id":"jeppep0j1cguag5v53l4lvq49","mutable":true,"inUse":false,"size":8192,"createdAt":"2022-07-31T06:57:07.579255711Z","lastUsedAt":"2022-07-31T06:57:13.861316848Z","usageCount":1,"parents":null,"description":"local source for dockerfile","recordType":"source.local","shared":false},{"id":"ur25i8ylum9326x9nas2lfyej","mutable":true,"inUse":false,"size":4096,"createdAt":"2022-07-31T06:57:07.580579468Z","lastUsedAt":"2022-07-31T06:57:13.860030835Z","usageCount":1,"parents":null,"description":"local source for context","recordType":"source.local","shared":false}]

$ buildctl prune --format '{{json .}}'
{"id":"u0nmub83lksjb9qu04y23v48m","mutable":true,"inUse":false,"size":2805760,"createdAt":"2022-07-31T06:57:11.68658936Z","lastUsedAt":"2022-07-31T06:57:13.858709691Z","usageCount":1,"parents":["s1d9pk3439ap6e5tt67bndctk"],"description":"mount / from exec /bin/sh -c apk add --no-cache neofetch","recordType":"","shared":false}
{"id":"jeppep0j1cguag5v53l4lvq49","mutable":true,"inUse":false,"size":8192,"createdAt":"2022-07-31T06:57:07.579255711Z","lastUsedAt":"2022-07-31T06:57:13.861316848Z","usageCount":1,"parents":null,"description":"local source for dockerfile","recordType":"","shared":false}
{"id":"ur25i8ylum9326x9nas2lfyej","mutable":true,"inUse":false,"size":4096,"createdAt":"2022-07-31T06:57:07.580579468Z","lastUsedAt":"2022-07-31T06:57:13.860030835Z","usageCount":1,"parents":null,"description":"local source for context","recordType":"","shared":false}
{"id":"s1d9pk3439ap6e5tt67bndctk","mutable":false,"inUse":false,"size":8979670,"createdAt":"2022-07-31T06:57:11.046458864Z","lastUsedAt":"2022-07-31T06:57:13.862022729Z","usageCount":1,"parents":null,"description":"pulled from docker.io/library/alpine@sha256:7580ece7963bfa863801466c0a488f11c86f85d9988051a9f9c68cb27f6b7872","recordType":"","shared":false}

$ buildctl debug info --format '{{json .}}'
{"buildkitVersion":{"package":"github.com/moby/buildkit","version":"v0.10.0-380-g874eef9b","revision":"874eef9b70dbaf4f074d2bc8f4dc64237f8e83a0"}}

[tonistiigi]

Do we need this? I think the examples in the PR work without this as well.

[AkihiroSuda]

This is not new in this PR, just moved from cmd/buildctl/debug/workers.go
405565c