oras push of oci.artifact.manifest to use default of application/tar

[SteveLasker]

When using the new oci.artifact.manifest, propose we fixup the default behavior for creating blobs.
Today, we create:

  "mediaType": "application/vnd.oci.image.layer.v1.tar",
  "digest": "sha256:204e7c423c891d0e4b057c4ecb068a53ffc991ef5a3bb47467f1b8088775dc48",
  "size": 84,
  "annotations": {
    "org.opencontainers.image.title": "sbom.json"
  }

Referencing issue #178 to default to:

  "mediaType": "application/tar",
  "digest": "sha256:204e7c423c891d0e4b057c4ecb068a53ffc991ef5a3bb47467f1b8088775dc48",
  "size": 84

[shizhMSFT]

@SteveLasker looks like this issue is outdated. Please re-open and add up-to-date details if needed.