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/api/src/main/kotlin/io/element/android/libraries/mediaplayer/api/MediaPlayer.kt

@@ -46,6 +46,12 @@ interface MediaPlayer : AutoCloseable {
      */
     fun seekTo(positionMs: Long)
 
+    /**
+     * Sets the playback speed.
+     * @param speed The playback speed (e.g., 0.5f for half speed, 1.0f for normal, 2.0f for double speed)
+     */
+    fun setPlaybackSpeed(speed: Float)
+
     /**
      * Releases any resources associated with this player.
      */