Cross-registry blob mount support

[wjordan]

#275 contains a spec change that seems to have narrowed the scope of cross-repository blob mounting by adding the words within the same registry to the description of the API call.

Looking back at the discussion of the original spec proposal in distribution/distribution#908 and the discussion leading up to it in distribution/distribution#634, it appears that restricting the spec to only allow for same-registry mounting was specifically avoided in order to intentionally allow for the future possibility of cross-registry mounting implementations/extensions, along with a mention that "the code should be designed so that cross-registry-cross-repository can be added in the future".

I couldn't find any more recent public discussion around the spec change in that PR, so it's unclear to me if the added text was an intentional change based on private discussions (in which case I would be interested in a public summary of these discussions), or an accidental re-interpretation of the original spec's intentions (in which case it would make sense to revert the change).

This issue is newly significant because Google has been working on an implementation of cross-registry mounting in their registry (see google/go-containerregistry#1321) and they expressed an interest in having their approach standardized moving forward.

[jonjohnsonjr]

seems to have narrowed the scope of cross-repository blob mounting

@sargun I can't tell from the GitHub discussion -- was this an intentional narrowing?