Support sending gallery messages behind a feature flag

Adds the ability to send a gallery message (multiple attachments in one message)
via the media upload preview, wired through the timeline controller/proxy to the
SDK's sendGallery. Gated by a `galleryEnabled` developer flag (default off),
matching Element X Android — with the flag off, attachments are sent as
individual messages as before.

Extends the community PR #5601, rebased onto develop. Tracking:
element-hq/element-meta#3201

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

ElementX/Sources/Services/Timeline/TimelineController/TimelineController.swift

@@ -402,6 +402,16 @@ class TimelineController: TimelineControllerProtocol {
         }
     }
     
+    func sendGallery(itemInfos: [GalleryItemInfo],
+                     caption: String?,
+                     formattedCaption: String?,
+                     inReplyToEventID: String?) async -> Result<Void, TimelineControllerError> {
+        await activeTimeline.sendGallery(itemInfos: itemInfos,
+                                         caption: caption,
+                                         formattedCaption: formattedCaption,
+                                         inReplyToEventID: inReplyToEventID).mapError(TimelineControllerError.timelineProxyError)
+    }
+    
     // MARK: - Polls
     
     func createPoll(question: String, answers: [String], maxSelections: Int, pollKind: Poll.Kind) async -> Result<Void, TimelineControllerError> {