PDC Agent: Native Go SSH Support

The PDC agent now supports a native Go SSH mode, enabled with the -use-gossh flag. This replaces the agent’s reliance on an external OpenSSH process with a direct SSH implementation via Go’s golang.org/x/crypto/ssh library, and brings several improvements:

1. Better concurrency within a single agent process. The OpenSSH client process is single-threaded and historically pdc-agent has worked around that limitation with multiple connections (-connections). The -use-gossh implementation does not have this limitation and therefore pdc-agent can take better advantage of the host hardware.
2. More accurate metrics. Metrics are now captured in-process rather than parsed from OpenSSH logs, resulting in more reliable observability data.
3. Better connection cleanup and error handling. pdc-agent can now send proper cancellation requests and also receive error codes directly.
4. Reduced complexity. The agent no longer needs to spawn or manage an external process, implementing only the SSH functionality required for TCP/IP forwarding.

To enable: Pass the -use-gossh flag when starting the agent.

Note: -use-gossh currently ignores -connections and most -ssh-flag values. However, -permit-domains is another new flag to use with -use-gossh to restrict domain forwarding. There is a “best effort” attempt to support domain restrictions specified in the -ssh-flag style.