Add SSH Agent Support

[helje5]

It would be nice to have SSH Agent support in the lib, I could use this right now :-)

Opening a separate issue to #94, which is about files, but mentions the topic already:

I think the most useful addition to work around this would be to add support for the ssh agent protocol, and use that as a potential backing for NIOSSHPrivateKey. This is admittedly a lot of work, but it circumvents the need for us to manage the protection of the key at all, and encourages good SSH key hygiene.

[Lukasa]

I think adding the support for the SSH agent protocol is a great idea. We'd welcome that contribution.

[camunro]

Friendly ping on #220. This PR is intentionally minimal and aimed at unblocking ssh-agent integration via external signer hooks, with tests/docs included.