Split VoiceMessageType from AudioMessageType (#1664)

Currently, for compatibility reasons, we implement MSC3245v1 which puts the voice data inside an audio message type. Though at times it seems impractical to deal with a single message type which effectively represents 2 different kinds of messages.

This PR creates a new message type called `VoiceMessageType` which is used whenever we receive an event with `"msgtype": "m.audio"` which also has the `"org.matrix.msc3245.voice": {}` field. This makes it easier to process voice messages as different entities throughout the rest of the codebase.

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/timeline/item/event/MessageType.kt

@@ -48,8 +48,13 @@ data class AudioMessageType(
     val body: String,
     val source: MediaSource,
     val info: AudioInfo?,
+) : MessageType
+
+data class VoiceMessageType(
+    val body: String,
+    val source: MediaSource,
+    val info: AudioInfo?,
     val details: AudioDetails?,
-    val isVoiceMessage: Boolean,
 ) : MessageType
 
 data class VideoMessageType(