libobs: Implement obs_service func to get max bitrates

(This commit also modifies rtmp-services)

Implements obs_service_get_max_bitrate, which allows retrieving the
maximum audio/video bitrates directly rather than being forced to use
the apply method. Makes it a bit easier to get the bitrate values.

libobs/obs.h

@@ -2234,6 +2234,9 @@ EXPORT const char *obs_service_get_id(const obs_service_t *service);
 EXPORT void obs_service_get_max_res_fps(const obs_service_t *service, int *cx,
 					int *cy, int *fps);
 
+EXPORT void obs_service_get_max_bitrate(const obs_service_t *service,
+					int *video_bitrate, int *audio_bitrate);
+
 /* NOTE: This function is temporary and should be removed/replaced at a later
  * date. */
 EXPORT const char *obs_service_get_output_type(const obs_service_t *service);