feat: add playback gain methods to all platforms

ios/Classes/LinphoneBridge.swift

@@ -280,6 +280,14 @@ class LinphoneBridge {
     func getMicGain() -> Float {
         return core.micGainDb
     }
+
+    func setPlaybackGain(level: Float) {
+        core.playbackGainDb = level
+    }
+
+    func getPlaybackGain() -> Float {
+        return core.playbackGainDb
+    }
 }
 
 // MARK: - CoreDelegate