Should we make adding a new mediaType easier

[sajayantony]

As a part of the OCI reference type work I have been trying to use the oras library to push a reference artifact.
Upon using CopyGraph I was kind of stumped why the manifest was existing even though it was not pushed as one -

oras-go/copy.go

Lines 81 to 84 in ed1a257

	exists, err := dst.Exists(ctx, desc)
	if err != nil {
	return nil, err
	}

I realized that the descriptor was pushed as blob and not as a manifest when debugging the above it failed to identify the descriptor as a manifest.

On a side note I don't know how this got stored in this API. As a blob or manifest when not adding the new manifest type.

	// store manifest
	if err := store.Push(ctx, manifestDesc, bytes.NewReader(manifestBytes)); err != nil && !errors.Is(err, errdef.ErrAlreadyExists) {
		return ocispec.Descriptor{}, fmt.Errorf("failed to push manifest: %w", err)
	}

Then I realized I could pass in the set of manifest types into the repo client. But this made adding a new mediaType by requiring the full set of types to be defined.
Should we support method on Repo like DefaultManifestMediaTypes() which would provide the set of string that we use as defaults.

This would enable adding a media type like this

repo.ManifestMediaTypes = append(repo.DefaultManifestMediaTypes(), ""application/vnd.oci.artifact.manifest.v1+json"

I would also like to discuss how can we push a manifest as a blob if the mediaType is used to distinguish whether its a blob or manifest.

/cc @Wwwsylvia @shizhMSFT