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/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 }