Support private registries for Maven

[cuixq]

Now by default, deps.dev API is queried for package versions and requirements when resolving dependencies.

We should also support fetching these data from private registries as well.

[jenspopp]

That would be a highly appreciated feature ;-) Right now it already fails for my most simple projects with:

failed extracting pom.xml: failed to merge parents: failed to get Maven project my.maven:super-pom:1.7.2: API query failed: Maven registry query status: 4 04 Not Found

[cuixq]

Some updates regarding this issue:

• There is a MavenRegistryAPIClient to talk to a Maven registry directly for metadata. Currently the URL is hard-coded to Maven Central, and we would like to make this configurable.
• There is also MavenRegistryClient which can be used as a DependencyClient in dependency resolution. More work is needed to make it work with multiple-registry resolution which may involve working on deps.dev's Maven resolver.

[another-rex]

Should this issue be closed with #1286 ?

[cuixq]

There is one optimisation PR in progress and this can be closed after that.