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.
Mauro
b5b28b9986