build: set record provenance in response

[crazy-max]

follow-up discussion with @tonistiigi
related to #1681

Previously we got containerimage.buildinfo in the metadata file but buildinfo has been removed in BuildKit 0.12 (moby/buildkit#3582): https://github.com/moby/buildkit/blob/master/docs/deprecated.md#build-information

This was useful to see all sources that were used by the build with their exact versions and also the configuration that was passed to the build.

However with the provenance attestation supported since BuildKit v0.11, we have similar (and a "bit" more) information available.

With this change we set the provenance saved along the build record in the metadata file similar to containerimage.buildinfo that is named buildx.build.provenance. Will set minimal provenance by default but can be customized using the BUILDX_METADATA_PROVENANCE env var.

---

$ cd buildx/
$ BUILDX_METADATA_PROVENANCE=max docker buildx --builder builder bake binaries --metadata-file md.json
...
#19 copying files 56.29MB 0.3s done
#19 DONE 0.3s

#20 resolve build record provenance
#20 DONE 0.0s

md.json

---

$ cd buildx/
$ BUILDX_METADATA_PROVENANCE=max docker buildx --builder builder bake binaries-cross --metadata-file md.json
...
#28 copying files darwin/amd64 63.37MB 2.8s done
#28 copying files darwin/arm64 63.25MB 2.8s done
#28 DONE 2.8s

#29 resolve build record provenance
#29 DONE 0.0s

md.json

---

$ cd buildkit/
$ BUILDX_METADATA_PROVENANCE=max docker buildx --builder builder bake binaries --metadata-file md.json
...
#89 [integration-tests-base 18/18] COPY --link --from=binaries / /usr/bin/
#89 merging 1.8s done
#89 DONE 1.9s

#90 [integration-tests 1/1] COPY . .
#90 DONE 1.5s

#91 resolve build record provenance
#91 DONE 0.1s

md.json

---

[crazy-max]

@tonistiigi Updated to strip buildConfig and metadata from provenance if BUILDX_METADATA_PROVENANCE sets to min. Mode max sets full provenance. Let me know if defaulting to min would be good. Atm it doesn't set provenance at all.