Container virtual registry: Generalize Maven shared context and examples and use for container virtual registries

In !204090 (merged), we saw that the following Maven code can be generalized and be reused for container virtual registries.

1. ee/spec/support/shared_contexts/maven_vreg_shared_context.rb - rename to virtual_registries_shared_context.rb and pass the virtual registry type (reference).
2. ee/spec/support/shared_examples/requests/api/virtual_registries/maven_packages_shared_examples.rb (reference).

Note: The following is moved to #570301 and best to be done after the upstreams API endpoints are done

Check if there are common tests that can be extracted from #569364 (closed) and also consider grouping tests: !204090 (comment 2746832375)