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/View/DeveloperOptionsScreen.swift

@@ -97,6 +97,11 @@ struct DeveloperOptionsScreen: View {
                         .foregroundStyle(.compound.textCriticalPrimary)
                 }
                 
+                Toggle(isOn: $context.galleryEnabled) {
+                    Text("Gallery messages")
+                    Text("Allows sending multiple media in a single message. Received galleries always render regardless of this setting.")
+                }
+                
                 Toggle(isOn: $context.jumpToReadMarkerEnabled) {
                     Text("Jump to unread")
                     Text("Adds a button to jump to the read marker, plus a presence dot on the scroll-to-bottom button when new messages arrive while scrolled away.")