Add feature flag for voice messages (#1507)

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -43,4 +43,10 @@ enum class FeatureFlags(
         title = "Show notification settings",
         defaultValue = true,
     ),
+    VoiceMessages(
+        key = "feature.voicemessages",
+        title = "Voice messages",
+        description = "Send and receive voice messages",
+        defaultValue = false,
+    ),
 }