Add support for SOCKS5 UDP datagram wrapping / unwrapping

[sethmlarson]

This is the most interesting to implement in a sans-I/O fashion while also supporting downstream consumers like aioquic.

[sethmlarson]

cc: @jlaine

https://github.com/sethmlarson/socksio/blob/master/socksio/socks5.py#L272

--> 332 raise NotImplementedError() # pragma: nocover