Support for Maven Central

[pierre]

• Write a new version of NexusApiCallsV2 class, to support Maven Central
• Keep NexusApiCallsV2 in use when oss.sonatype.org is specified, otherwise default to NexusApiCallsV2
• Change all defaults from https://oss.sonatype.org to https://repo1.maven.org (this is the new default)
• The implementation of the new class must support similar calls as NexusApiCallsV2. Use https://central.sonatype.org/search/rest-api-guide/ for the search API.
• Add new remote spec to verify Kill bill 0.24.15 can be pulled from Maven Central. KPM::Installer.get_kb_latest_stable_version should report 0.24.15 today (add a test that it's >= 15).
• Once the change is made, docker_ci with the patch 54418b5 should pass. It's failing today: https://github.com/killbill/killbill-cloud/actions/runs/16506627013