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/Screens/Settings/DeveloperOptionsScreen/DeveloperOptionsScreenModels.swift

@@ -67,6 +67,8 @@ protocol DeveloperOptionsProtocol: AnyObject {
     
     var linkPreviewsEnabled: Bool { get set }
     
+    var galleryEnabled: Bool { get set }
+    
     var jumpToReadMarkerEnabled: Bool { get set }
     
     var linkNewDeviceEnabled: Bool { get set }