Implement ssh-agent forwarding for git remotes

* New forward_ssh_agent feature and parameter can be used with git provider
* New feature depends on ssh_identity feature
* Add specs for git provider's ssh_identity implementation
* Add specs for new git provider functionality

Motivation:
This functionality allows gitolite's upstream functionality to be used as a read-through proxy for git remotes, although there may be other, unforeseen use cases. This change is backwards-compatible.