This PR adds validations for the new new funnel/serve
commands under the following rules:
1. There is always a single config for one port (bg or fg).
2. Foreground configs under the same port cannot co-exists (for now).
3. Background configs can change as long as the serve type is the same.
Updates #8489
Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>