Proposal: Pushing multiple tags in single push

[jcarter3]

The current PUT manifest implementation is inefficient when multiple tags need updated to point to the same digest. Additionally, the current spec does not have a solution when the manifest does not use sha256 (upload manifest with blake3, then push tag, it'd hash as sha256)

I propose updating the PUT manifest to allow for multiple tags:

PUT /v2/<name>/manifests/<digest>?tag=v3&tag=v3.1&tag=v3.1.2&tag=latest

The response from the registry will include a header OCI-Tags=v3,v3.1,v3.1.2,latest to confirm which tags the registry successfully created.

This proposal:

1. Allows for more efficient tag creation paths when multiple tags need created
2. Allows specifying hashing algorithm for a given tag
3. Easy opt-in for registries -> if OCI-Tags is not returned, clients fall back to existing tag push.

While there are solutions here that would be more "pure" with regards to HTTP specs (maybe doing a PATCH for tags), I view this solution has having less friction to implement.

[sudo-bmitch]

For an alternative to sha512 support, see #543. There's a discussion for sha512 support in #494.