Add global Downsampling feature (#1575)

* Add global downsampling feature

* Default to Opus &  consider player transcoder

* Add a test case for DefaultDownsamplingFormat

Co-authored-by: Deluan <deluan@navidrome.org>

core/media_streamer.go

@@ -142,6 +142,10 @@ func selectTranscodingOptions(ctx context.Context, ds model.DataStore, mf *model
 			if p, ok := request.PlayerFrom(ctx); ok {
 				cBitRate = p.MaxBitRate
 			}
+		} else if reqBitRate > 0 && conf.Server.DefaultDownsamplingFormat != "" {
+			// If no format is specified and no transcoding associated to the player, but a bitrate is specfied, and there is no transcoding set for the player, we use the default downsampling format
+			log.Debug("Default Downsampling", "Using default downsampling format", conf.Server.DefaultDownsamplingFormat)
+			cFormat = conf.Server.DefaultDownsamplingFormat
 		}
 	}
 	if reqBitRate > 0 {