Feature request: Allow to configure additional upstreams of supported registry formats #295

It would be nice to be able to configure additional upstreams for the supported registry formats.

We make use of the Maven Central and https://repo.grails.org/grails/restricted in our build. The latter is also in Maven repository format. But currently it seems to be only possible to proxy a single of these URLs. I would like to proxy both. For example, support a config like the folllowing, similar to what seems already possible for Helm:

upstreams:
 maven:
 central: https://repo1.maven.org/maven2
 grails: https://repo.grails.org/grails/restricted

and then serve it under /maven/{name}/.