sessions: add developer command to kill SSH remote agent host

[Huynhthuongg]

sessions: add developer command to kill SSH remote agent host

Add a Developer: Kill Remote Agent Host (SSH)... command
(workbench.action.sessions.killRemoteAgentHostSSH) that lets developers
kill a wedged remote agent host process and tear down the SSH tunnel.

• Add killRemoteAgentHost(host) to ISSHRemoteAgentHostService and
  ISSHRemoteAgentHostMainService interfaces
• Implement in SSHRemoteAgentHostMainService using the existing
  cleanupRemoteAgentHost helper, then dispose the connection
• Proxy through the renderer-side service and add a no-op to the
  null browser implementation
• Register a Developer category action that quick-picks from active
  SSH connections and calls killRemoteAgentHost

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

Originally posted by Huỳnh Thương (@Huynhthuongg) in microsoft/vscode#322226 (comment)