UDP client side communication boilerplate for RADIUS

Part of the RADIUS redesign.

boilerplate for the RADIUS client
UDP client added to the RADIUS client code with timeouts and retries as in freeradius-client:lib/sendserver.c:339-400
unit test for sending and receiving basic UDP datagrams
- either with mock server as target endpoint, or with FreeRADIUS server but DISABLED_ by default in that latter case

Issue is considered resolved if the server receives a datagram, even if it considers it malformed, or ignores it silently.

Edited Sep 06, 2023 by Andrei Pavel

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