Distribution proposal: support reverse-lookup from a manifest to its referrers (such as an index)

[avtakkar]

With regards to the manifest referrer metadata store discussed in #14, two options come to mind for implementing the reverse lookup:

1. Let each manifest have a reverse lookup to all indexes that reference it. Retrieving referrer metadata for a particular manifest would mean first retrieving the indexes, and then following their references to the config objects to get the requested metadata references.

2. Let each manifest have a reverse lookup to all config objects referenced by indexes that reference the manifest. This approach short circuits the indexes from the reverse lookup, providing direct access to the referrer metadata.

[SteveLasker]

Closing as this is being tracked through the proposed reference type working group, with the current design tracked through oci.artifact.manifest PR #29