MOB-2213: Add information about poi count to map CRUD API

@bkoval Pretty much either one works for me. Let's clarify: I need to display all maps for particular one wiki with information about number of pins associated with each map. So.. if I add map_id as a filter parameter I will have to call the API method <number of maps> times. If I add city_id as a filter parameter I will have to call it only once and then do some lightweight processing in the client. Please advice me which one is more aligned with your (the team) preference.
(btw. it is something that we might redo or reconsider in the future, after we have more data about how maps, and maps in VE are being used)