Would you be open to a contribution that adds Maven backend support?

I think the base work here is:

• add a new Maven implementation that's similar to the Artifactory one but uses maven-metadata.xml instead of Artifactory queries
• likely pulling out a Registry trait for both to implement
• plumbing to allow specifying a registry is Artifactory or Maven, maintaining Artifactory as a default

There's some design decision questions here, in particular how the manifest should store the registry type. A couple of options on that front:

• take a crates style approach and embed "maven+" or "artifactory+" in front of a registry URI to encode type (with absence defaulting to artifactory)
• use a nested type that encodes type/URL separately
• whether this deserves a new rev of the manifest or if it should be handled transparently