core: option to run client as chat relay; cli api to get, set relays (#6407)

src/Simplex/Chat/Mobile.hs

@@ -253,6 +253,7 @@ mobileChatOpts dbOptions =
             logFile = Nothing,
             tbqSize = 4096,
             deviceName = Nothing,
+            chatRelay = False,
             highlyAvailable = False,
             yesToUpMigrations = False,
             migrationBackupPath = Just ""