[FEATURE] Provide JGit `SshSessionFactory` via `setTransportConfigCallback`

[fmeum]

Is your feature request related to a problem? Please describe.
We currently set a static SshSessionFactory, which opens us up to leaks.

Describe the solution you'd like
Use setTransportConfigCallback instead.

Describe alternatives you've considered
Fine as it is now, but could be better.

Additional context
Blocked on #995.