Add SSH agent forwarding support

[Wackymax]

Summary

• add SSH agent forwarding support via OpenSSH auth-agent requests
• expose SSHAgentHandler + SSHKeyPairAgent for in-memory key signing
• handle auth-agent@openssh.com channels when enabled
• honor RSA sign-request flags to support rsa-sha2-256/512 and ssh-rsa

Notes

• agent forwarding is only enabled when the client provides an agent handler
• requests are handled in-memory; no keys are written to disk

Testing

• not run (library-only change)

[vicajilau]

Sorry the delay addessing this. Could you review ssh_agent dart format? Pipeline is not passing @Wackymax

[vicajilau]

I am closing this task in favor of #144