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/TimelineProxyProtocol.swift

@@ -110,6 +110,11 @@ protocol TimelineProxyProtocol: Sendable {
                           waveform: [Float],
                           requestHandle: @MainActor (SendAttachmentJoinHandleProtocol) -> Void) async -> Result<Void, TimelineProxyError>
     
+    func sendGallery(itemInfos: [GalleryItemInfo],
+                     caption: String?,
+                     formattedCaption: String?,
+                     inReplyToEventID: String?) async -> Result<Void, TimelineProxyError>
+    
     func sendReadReceipt(for eventID: String, type: ReceiptType) async -> Result<Void, TimelineProxyError>
     func markAsRead(receiptType: ReceiptType) async -> Result<Void, TimelineProxyError>