JGit: Support interaction with an SSH agent

### Affected version

2.1.0

### Bug description

The bundle fragment https://github.com/eclipse-jgit/jgit/blob/master/org.eclipse.jgit.ssh.apache.agent provides support for communicating with an SSH agent. This should be included in the JGit provider to allow leveraging keys from SSH agents.

Unfortunately this has been introduced with JGit 6 (requiring Java 11): https://github.com/eclipse-jgit/jgit/commit/634302d2da74226cff9f78e121ad5b8216c476e6

Start by reviewing the JGit SSH agent bundle fragment linked in the issue and the Maven SCM JGit provider integration. Check how the provider handles the Java version constraint, then verify that SSH-agent keys can be used without requiring unsupported JGit or Java versions.

Written by the indexing model from the issue text.