feat: add playback gain methods to all platforms

android/src/main/kotlin/xyz/nuark/liblinphone_flutter/LiblinphoneFlutterPlugin.kt

@@ -227,6 +227,21 @@ class LiblinphoneFlutterPlugin : FlutterPlugin, ActivityAware, MethodCallHandler
                 result.success(linphoneBridge.getMicGain())
             }
 
+            "setPlaybackGain" -> {
+                try {
+                    val level = call.argument<Float>("level")!!
+                    linphoneBridge.setPlaybackGain(level)
+                    result.success(true)
+                } catch (e: Exception) {
+                    Log.e(TAG, "setPlaybackGain: ${e.message}")
+                    result.error("error", e.message, e)
+                }
+            }
+
+            "getPlaybackGain" -> {
+                result.success(linphoneBridge.getPlaybackGain())
+            }
+
             else -> {
                 result.notImplemented()
             }

android/src/main/kotlin/xyz/nuark/liblinphone_flutter/LinphoneBridge.kt

@@ -384,4 +384,12 @@ class LinphoneBridge(
     fun getMicGain(): Float {
         return core.getMicGainDb()
     }
+
+    fun setPlaybackGain(level: Float) {
+        core.setPlaybackGainDb(level)
+    }
+
+    fun getPlaybackGain(): Float {
+        return core.getPlaybackGainDb()
+    }
 }