[DNSSD] Service Discovery Pt I - Req service & cb with SIN
- Fixed 2 TICS warnings
- Added api 'pico_mdns_getservicename'
- Added call/cb for "_printer._tcp.local" in example app

TODO
- Request SRV (and TXT) packets based on SIN
- Return received SRV & TXT data with cb in a useful way