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.xcodeproj/project.pbxproj

@@ -7610,16 +7610,16 @@
 			projectDirPath = "";
 			projectRoot = "";
 			targets = (
-				C0FAEB81CFD9776CD78CE489 /* ElementX */,
-				FEB53A5BC378C913769656D8 /* NSE */,
-				19F0C845D67E9BEA4BE7133E /* ShareExtension */,
-				676A87694F0F247B2AF1A142 /* MatrixRustSDKMocks */,
-				32C23C8D224D46EFE62AFAD0 /* UnitTests */,
-				7A17BE29BAC81ADBAC6349D9 /* PreviewTests */,
-				0E28CD62691FDBC63147D5E3 /* UITests */,
-				D3DB351B7FBE0F49649171FC /* IntegrationTests */,
 				C0C687DE1D270F9895FEE186 /* AccessibilityTests */,
+				C0FAEB81CFD9776CD78CE489 /* ElementX */,
+				D3DB351B7FBE0F49649171FC /* IntegrationTests */,
+				676A87694F0F247B2AF1A142 /* MatrixRustSDKMocks */,
+				FEB53A5BC378C913769656D8 /* NSE */,
 				F8E276FD6DC43EADB85241BC /* Periphery */,
+				7A17BE29BAC81ADBAC6349D9 /* PreviewTests */,
+				19F0C845D67E9BEA4BE7133E /* ShareExtension */,
+				0E28CD62691FDBC63147D5E3 /* UITests */,
+				32C23C8D224D46EFE62AFAD0 /* UnitTests */,
 			);
 		};
 /* End PBXProject section */