Add PreSharedKey extensions for DTLS 1.3

[theodorsm]

Description

DTLS 1.3 uses two new extensions to negotiate PreSharedKey (PSK) mode.

Progress:

• pre_shared_key extension
• pre_shared_key tests and fuzz
• psk_exchange_modes extension
• psp_exchange_modes tests and fuzz

Reference issue

Fixes #771

[codecov]

Codecov Report

❌ Patch coverage is 47.12644% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.14%. Comparing base (9121462) to head (1cf17bb).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/protocol/extension/psk_key_exchange_modes.go	0.00%	28 Missing ⚠️
pkg/protocol/extension/pre_shared_key.go	69.49%	9 Missing and 9 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #773      +/-   ##
==========================================
- Coverage   81.40%   81.14%   -0.27%     
==========================================
  Files         103      107       +4     
  Lines        5717     5924     +207     
==========================================
+ Hits         4654     4807     +153     
- Misses        684      725      +41     
- Partials      379      392      +13     

Flag	Coverage Δ
go	81.14% <47.12%> (-0.27%)	⬇️

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