Add DNS-SD command

[gpotter2]

This PR:

• adds a verry simple dnssd() command to perform mDNS service discoveries. I like it
• By default it performs a rfc6763 type-enumeration (section 9), but you can also try services like _spotify-connect._tcp.local, etc. the .show() command then tries to summarize the results

Note:

• this is mostly unusable without the work that has been done to improve multicast support in Support for Scope Identifiers in IP addresses for multicast/link-local support #4461
• this is unusable on Windows without Fix sr() on multiple interfaces and libpcap L3 sockets #4474

meaning this sadly probably won't get merged before 2.6

[codecov]

Codecov Report

Attention: Patch coverage is 20.45455% with 35 lines in your changes missing coverage. Please review.

Project coverage is 81.65%. Comparing base (9738e4a) to head (82a07b4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4481      +/-   ##
==========================================
- Coverage   81.68%   81.65%   -0.03%     
==========================================
  Files         355      355              
  Lines       84702    84745      +43     
==========================================
+ Hits        69186    69197      +11     
- Misses      15516    15548      +32     

Files	Coverage Δ
scapy/layers/dns.py	83.99% <20.45%> (-3.69%)	⬇️

... and 5 files with indirect coverage changes