DTLS 1.3 ACK message

[theodorsm]

Description

This adds the new ACK message for DTLS 1.3 with ContentType, Marshal and Unmarshal. Message structure is defined in RFC 9147 Appendix A.3.

The ACK messages are needed to implement the DTLS 1.3 handshake.

Reference issue

Related to #818

[codecov]

Codecov Report

❌ Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.60%. Comparing base (d0e736c) to head (862beac).

Files with missing lines	Patch %	Lines
pkg/protocol/ack.go	80.95%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #819      +/-   ##
==========================================
- Coverage   82.66%   82.60%   -0.07%     
==========================================
  Files         121      122       +1     
  Lines        6853     6874      +21     
==========================================
+ Hits         5665     5678      +13     
- Misses        777      783       +6     
- Partials      411      413       +2     

Flag	Coverage Δ
go	82.60% <80.95%> (-0.07%)	⬇️

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