Support sending gallery messages behind a feature flag (#5909)

dds sending of multi-media "gallery" messages, gated behind the new galleryEnabled developer option (Developer Options → Room, off by default). Rendering of received galleries ships in a separate PR.

Wires sendGallery through TimelineProxy/TimelineController onto the SDK's Timeline.sendGallery.
MediaUploadPreviewScreen sends selected items as one gallery when the flag is on.
Co-authored with the original author of #5601.

NOTE: This PR allows to send gallery messages, but they won't be rendered without the gallery timeline item PR.

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

@@ -402,6 +402,14 @@ class TimelineController: TimelineControllerProtocol {
         }
     }
     
+    func sendGallery(itemInfos: [GalleryItemInfo],
+                     caption: String?,
+                     inReplyToEventID: String?) async -> Result<Void, TimelineControllerError> {
+        await activeTimeline.sendGallery(itemInfos: itemInfos,
+                                         caption: caption,
+                                         inReplyToEventID: inReplyToEventID).mapError(TimelineControllerError.timelineProxyError)
+    }
+    
     // MARK: - Polls
     
     func createPoll(question: String, answers: [String], maxSelections: Int, pollKind: Poll.Kind) async -> Result<Void, TimelineControllerError> {