Support for configuring a store URL

Also requires updates for polycentric-core, polycentric-server, polycentric-android, harbor-web.

harbor-flutter 13a2dc8c

polycentric-core polycentric@6658d8e1

polycentric-server polycentric@bef4fb31

polycentric-android videostreaming/polycentricandroid@84e2a4ce

closed