DTLS 1.3: Add flight 3

[theodorsm]

Description

This PR aims to implement flight 3 (sending second CH and parsing SH) for DTLS 1.3.

Blocked by #738 (copied code / commits)

Reference issue

Partly fixes #825

[codecov]

Codecov Report

❌ Patch coverage is 48.44075% with 248 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.15%. Comparing base (179c353) to head (dd3cba2).

Files with missing lines	Patch %	Lines
flighthandlers_client_13.go	37.42%	92 Missing and 10 partials ⚠️
conn.go	59.71%	56 Missing and 29 partials ⚠️
flight_13.go	14.28%	36 Missing ⚠️
handshaker_13.go	51.51%	16 Missing ⚠️
flighthandler_13.go	0.00%	8 Missing ⚠️
util.go	93.75%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (48.44%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #834      +/-   ##
==========================================
- Coverage   82.35%   80.15%   -2.20%     
==========================================
  Files         123      127       +4     
  Lines        6999     7439     +440     
==========================================
+ Hits         5764     5963     +199     
- Misses        820     1021     +201     
- Partials      415      455      +40     

Flag	Coverage Δ
go	80.15% <48.44%> (-2.20%)	⬇️

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