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>

conf/configuration.go

@@ -67,6 +67,9 @@ type configOptions struct {
 	Spotify      spotifyOptions
 	ListenBrainz listenBrainzOptions
 
+	//test downsampling
+	DefaultDownsamplingFormat string
+
 	// DevFlags. These are used to enable/disable debugging and incomplete features
 	DevLogSourceLine           bool
 	DevLogLevels               map[string]string
@@ -268,6 +271,8 @@ func init() {
 	viper.SetDefault("listenbrainz.enabled", true)
 	viper.SetDefault("listenbrainz.baseurl", "https://api.listenbrainz.org/1/")
 
+	viper.SetDefault("defaultdownsamplingformat", consts.DefaultDownsamplingFormat)
+
 	// DevFlags. These are used to enable/disable debugging and incomplete features
 	viper.SetDefault("devlogsourceline", false)
 	viper.SetDefault("devautocreateadminpassword", "")