feat: WIP: new friend finding

[zugz]

Implementing https://github.com/zugz/tox-DHTAnnouncements

For the purposes of the CLA, this PR is not a Contribution.

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This change is

[codecov]

Codecov Report

Merging #1335 into master will decrease coverage by 0.1%.
The diff coverage is 96.9%.

@@           Coverage Diff            @@
##           master   #1335     +/-   ##
========================================
- Coverage    84.6%   84.4%   -0.2%     
========================================
  Files          86      89      +3     
  Lines       15777   16053    +276     
========================================
+ Hits        13349   13551    +202     
- Misses       2428    2502     +74

Impacted Files	Coverage Δ
toxcore/TCP_client.c	63.1% <100%> (-1.7%)	⬇️
toxcore/network.c	82% <100%> (+0.1%)	⬆️
toxcore/TCP_server.c	80.6% <100%> (-2.1%)	⬇️
toxcore/TCP_connection.c	78.7% <100%> (+1.2%)	⬆️
toxcore/net_crypto.c	94.7% <100%> (-0.3%)	⬇️
toxcore/forwarding.c	100% <100%> (ø)
toxcore/onion.c	98% <60%> (-0.8%)	⬇️
toxcore/onion_client.c	96% <80%> (-0.3%)	⬇️
toxcore/DHT.c	76.8% <88.4%> (-0.9%)	⬇️
auto_tests/forwarding_test.c	97.6% <97.6%> (ø)
... and 17 more

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