Determine registry URI format

[theory]

Determine the URI format for the OCI registry. Things to consider:

• Will it be used for non-PGXN/Trunk images?
• Where should extensions built from the core live?
• Should we prefix PGXN-built extensions with pgxn to distinguish from other sources?
• Should we expect to allow PGXN users to publish their own eventually?
• What does the UX look like for users of the CLI?
• How does it impact living all available packages?
• Is it possible to make an API call to list all PGXN and core-provided extensions?
• Or should core-provided extensions built by the PGXN build process use the same prefix as other PGXN distributions, even though core extension sources are not published on PGXN?

Done when a draft has been written up, submigged as an RFC pull request, and reviewed by members of the community.