cache/remotecache: normalize object cache layer media types

[crazy-max]

follow-up #6824

Updates remote cache chain marshaling so layer descriptors are normalized to OCI media types when descriptor media types are recorded.

This makes cache metadata written by the GHA, S3, and Azure Blob cache exporters store OCI layer media types in CacheLayer.Annotations. Registry and local cache exporters keep their existing exported manifest behavior through their oci-mediatypes option.

This doesn't add a new compatibility-version path because the affected cache metadata is internal to the remote cache backends.

[tonistiigi]

Not sure about this approach:

• Afaics the registry/local cache already defaulted to OCI. So not sure if this is changing anything about them. This seems to handle gha/s3/azblob, but in there I think it is only relevant to the CacheLayer.Annotations. I don't think it would require configurability there as these should be pretty much internal annotations.
• Compatibility-version is only for release artifact reproducibility. I don't think we should extend it for exported cache and the current versions were not handling it(probably not even possible to keep this as guarantee).

[crazy-max]

• Compatibility-version is only for release artifact reproducibility

Ah makes sense, I will get rid of compatibility changes.

[crazy-max]

@tonistiigi I reworked this to keep the change scoped to the object cache exporters.

Removed the compatibility version and removes the finalize option plumbing. Registry and local cache exporters are also back to their previous behavior, so their existing oci-mediatypes handling is unchanged.

The remaining change is only for GHA, S3, and Azure Blob cache exporters. They now serialize CacheLayer.Annotations.MediaType with OCI layer media types, since that metadata is internal to those cache backends.