Export cache to registry should support annotations

[neilime]

Description

Image tag created by cache-to with "registry" does not have given annotation.

It should be good to allows to pass some annotation to exported cache image. It is useful for many use cases, like setting image source so github can link the cache image package to the right repo (and permissions)...

buildctl build ... \
  --output type=image,name=localhost:5000/myrepo:image,push=true \
  --export-cache type=registry,ref=localhost:5000/myrepo:buildcache,manifest:org.opencontainers.image.source=https://github.com/my-org/my-repo \

Thanks