Enhanced resolve image config

[jedevc]

This implements the first option from #2944 :tada:

We extend ResolveImageConfigResponse to include a Manifest and an Index, while leaving the previous fields unchanged.

message ResolveImageConfigResponse {
	string Digest = 1 [(gogoproto.customtype) = "github.com/opencontainers/go-digest.Digest", (gogoproto.nullable) = false];
	bytes Config = 2;
	bytes Manifest = 3;
	bytes Index = 4;
}

Note

Digest may point to Manifest.Digest or Index.Digest or neither if schema1 is used - changing this behavior would be a backwards-incompatible change, and so is out-of-scope for this PR.

With this PR, we should be able to move the image resolution logic out of buildx imagetools, and into buildkit, which will allow respecting registry settings.