Support image-spec v1.1.0 and distribution-spec v1.1.0

[shizhMSFT]

Tasks:

• Support manifest with subject and index with subject #162
• Push Manifest with subject #159
• Delete Manifest with subject #160
• Implement Referrers API #161

References:

• image-spec
• distribution-spec

[pat-pan]

pat

[pat-pan]

@Wwwsylvia, I think this task can be split into 3 sub-tasks. Should we also split this issue into 3?

[Wwwsylvia]

@pat-pan Sure, please feel free to create the corresponding issues.

[Wwwsylvia]

Just realized that GetSuccessorsAsync did not handle subject.

oras-dotnet/src/OrasProject.Oras/Content/Extensions.cs

Lines 46 to 48 in f33af92

	var descriptors = new List<Descriptor>() { manifest.Config };
	descriptors.AddRange(manifest.Layers);
	return descriptors;