Add the gallery timeline item rendered as a placeholder

Introduces the GalleryRoomTimelineItem and its typed GalleryItem model (reusing the existing per-message content types), builds it in the RoomTimelineItemFactory and routes it through RoomTimelineItemView. For now it renders as a placeholder; the grid/list media views and the full-screen preview land in follow-up PRs.

Co-authored-by: svetoslav.mitov <svetoslav.mitov@sportinno.com>

ElementX/Sources/Services/Timeline/TimelineItems/EventBasedMessageTimelineItemProtocol.swift

@@ -18,6 +18,7 @@ nonisolated enum EventBasedMessageTimelineItemContentType: Hashable {
     case video(VideoRoomTimelineItemContent)
     case location(LocationRoomTimelineItemContent)
     case voice(AudioRoomTimelineItemContent)
+    case gallery(GalleryRoomTimelineItemContent)
 }
 
 nonisolated protocol EventBasedMessageTimelineItemProtocol: EventBasedTimelineItemProtocol {
@@ -27,7 +28,7 @@ nonisolated protocol EventBasedMessageTimelineItemProtocol: EventBasedTimelineIt
 nonisolated extension EventBasedMessageTimelineItemProtocol {
     var supportsMediaCaption: Bool {
         switch contentType {
-        case .audio, .file, .image, .video:
+        case .audio, .file, .image, .video, .gallery:
             true
         case .emote, .notice, .text, .location, .voice:
             false
@@ -48,6 +49,8 @@ nonisolated extension EventBasedMessageTimelineItemProtocol {
             content.caption
         case .video(let content):
             content.caption
+        case .gallery(let content):
+            content.caption
         case .emote, .notice, .text, .location, .voice:
             nil
         }
@@ -63,6 +66,8 @@ nonisolated extension EventBasedMessageTimelineItemProtocol {
             content.formattedCaption
         case .video(let content):
             content.formattedCaption
+        case .gallery(let content):
+            content.formattedCaption
         case .emote, .notice, .text, .location, .voice:
             nil
         }