Maven virtual registry: shareable upstreams

🔥 Problem

With #541115 (closed), we're going to implement a way to have multiple maven registries within a single top level group.

This situation will need that upstreams can be shared. Basically, the ability of adding an upstream to a maven virtual registry by referencing an existing upstream.

🚒 Solution

• The database structure should be already ready for that.
• The Rest API will need some updates.
  • Introduce an endpoint to list all the available maven upstreams given a top-level group.
  • Update the endpoint to add an upstream to a registry to accept only an upstream_id.
• Verify that the position update API (registry_upstream) is still working as expected.

This might need more than 1 MR.