Implement additional search backends

[quintesse]

The Maven Central search API) is very unreliable, it would be nice to have other options.

• Google https://docs.deps.dev/api/v3/ (test https://deps.dev/maven/org.codejive.jpm%3Ajpm)
• SMO https://github.com/apache/maven-indexer/tree/master/search-backend-smo
• RRB https://github.com/apache/maven-indexer/tree/master/search-backend-remoterepository
• There exists some internal undocumented CSC API , not sure it's a good idea trying to use that, though
• Others?

The idea would be to have an option to set the backend, simply --backend/-b or something?
With values like "google", "smo", "smo-csc".

And which would we make the default? "smo-csc" would probably be stable and up-to-date, but the Google one has really useful ordering, which the others lack. But it's not always the most up-to-date. Could we go crazy and search for artifacts using Google and then using smo-csc for versions?

[quintesse]

We now have a framework for adding more backends at least and one working alternative using central.sonatype.com. Unfortunately although the CSC service should be more reliable than the default SMO (search.maven.org) the downsides are slower response times and worse results, so I decided to keep SMO as the default for now.

The Google service is still an option, it has really good ordering of results (putting popular artifacts at the top) but it's search capabilities are pretty limited. So I'm not sure if it's worth implementing.

Edit: Added RRB to the issue description so there's at least a mention of it somewhere. It's advantage is that it works for private repositories because it will access them directly. The downside, no search to speak of, you either know the name of the thing you're looking for or you are out of luck.