PipeWireBaseEncodedStream: add pause() and resume()
Add pause()/resume(), which deactivate and reactivate the running stream via pw_stream_set_active() without tearing it down, keeping the produce thread, encoder, stream and fd alive. While paused no frames are delivered, so the encoder idles. This lets a consumer suspend encoding (for example when an RDP client minimizes and sends a Suppress Output PDU) and resume without re-establishing the PipeWire stream.