Feature: add room threads list (#6575)

Add threads list screen for rooms:

- Add `ThreadsListService` to subscribe to thread changes in the room.
- Create `ThreadsListView` and its associated node a presenters (the UI may change).
- Add a menu icon in the room screen to open it.

This is still pending info about unread threads, so several UI components related to it will be hidden.

* Add feature flag and use it to hide the access to this new screen

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Co-authored-by: ElementBot <android@element.io>

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/components/avatar/AvatarSize.kt

@@ -13,10 +13,12 @@ import androidx.compose.ui.unit.dp
 
 enum class AvatarSize(val dp: Dp) {
     CurrentUserTopBar(32.dp),
+    CurrentRoomTopBar(32.dp),
 
     IncomingCall(140.dp),
     RoomDetailsHeader(96.dp),
     RoomListItem(52.dp),
+    ThreadsListItem(52.dp),
 
     SpaceListItem(52.dp),