Virtual registries: add a test function to upstreams

🔥 Problem

Setting the upstream url and credentials will not guarantee that it will work nicely.

It would be nice to be able to test settings to make sure that they are correct. Similar to what we have in project integrations. See https://docs.gitlab.com/development/integrations/#define-configuration-test.

🚒 Solution

• Investigate how, given an url and credentials, we could send a request to a maven registry.
• From that investigation, expose a Rest API endpoint for such configuration test feature.