Add support for SSH credentials in reconciler and gitserver
Created by: eseliger
This is part 2 of campaigns SSH support, adding support in gitserver for private keys and not bailing out in the reconciler when an ssh credential is encountered.
Huge thanks to @LawnGnome for coming up with the idea to use an ssh-agent running in go, and the PoC code for it!
Closes https://github.com/sourcegraph/sourcegraph/issues/16895