Make config layers in ociremote mountable

[jonjohnsonjr]

The wrapping that oci.Signatures does hides the ConfigLayer() implementation in remote.Image that remote.Write relies on for determining if it can mount the config blob (vs having to re-upload it), so remote.Write is sending additional blob uploads, which incurs an additional roundtrip that we don't really need to do.

Explicitly implement ConfigLayer() in the wrappers by dispatching to the wrapped implementation fixes this.

Before this change, this check fails due to the wrapping:

https://github.com/google/go-containerregistry/blob/1b4e4078a545f2b6f96766a064b45ee77cdbefdd/pkg/v1/partial/with.go#L102-L104

Which means we get back this fallback implementation: https://github.com/google/go-containerregistry/blob/1b4e4078a545f2b6f96766a064b45ee77cdbefdd/pkg/v1/partial/with.go#L52-L88

Which means this check fails: https://github.com/google/go-containerregistry/blob/1b4e4078a545f2b6f96766a064b45ee77cdbefdd/pkg/v1/remote/write.go#L364

So we didn't attempt to mount.

https://go.dev/play/p/2kW3-XbdjmM

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 40.63%. Comparing base (2ef6022) to head (9432efd).
Report is 132 commits behind head on main.

Files	Patch %	Lines
pkg/oci/remote/image.go	0.00%	2 Missing ⚠️
pkg/oci/remote/signatures.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3741      +/-   ##
==========================================
+ Coverage   40.10%   40.63%   +0.52%     
==========================================
  Files         155      158       +3     
  Lines       10044    10179     +135     
==========================================
+ Hits         4028     4136     +108     
- Misses       5530     5543      +13     
- Partials      486      500      +14     

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