Support SSH keys as secrets

[rarkins]

Describe the proposed change(s).

Sometimes users have git dependencies - including submodules - which are authenticatable only using ssh and not https. We should find a way for users to provide a private SSH key to Renovate.

[viceice]

I think the easiest solution is to start the key agent, add the keys, process repo (pass key agent env as required), stop agent.

[bryansoftdev]

I think the easiest solution is to start the key agent, add the keys, process repo (pass key agent env as required), stop agent.

Would be nice to have this option available for the Mend-hosted apps too