Add variable playback speed feature for voice messages

Add playback speed control for voice messages with support for 0.5×, 1×, 1.5×, and 2× playback speeds. The speed button is displayed above the timestamp and cycles through the available speeds when tapped.

libraries/mediaplayer/test/src/main/kotlin/io/element/android/libraries/mediaplayer/test/FakeMediaPlayer.kt

@@ -95,6 +95,10 @@ class FakeMediaPlayer(
         }
     }
 
+    override fun setPlaybackSpeed(speed: Float) {
+        // no-op
+    }
+
     override fun close() {
         // no-op
     }