Add SSH Agent Support

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

Opening a separate issue to https://github.com/apple/swift-nio-ssh/issues/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.

Start by reviewing the NIOSSHPrivateKey integration and the SSH agent protocol, using linked issue 94 for context about key handling. Define the agent-backed authentication scope and how it should coexist with existing key support; done means the library can use an SSH agent without requiring direct private-key file management.

Written by the indexing model from the issue text.