DTLS 1.3: Add flights 0 and 1

[theodorsm]

Description

This PR aims to implement flight 0 (waiting server) and flight 1 (sending the first CH) for DTLS 1.3.

Blocked by #738 (copied code / commits)

Reference issue

Partly fixes #825

[codecov]

Codecov Report

❌ Patch coverage is 46.92443% with 302 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.72%. Comparing base (fd27a52) to head (5072f2d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
conn.go	59.71%	56 Missing and 29 partials ⚠️
flighthandlers_server_13.go	7.77%	80 Missing and 3 partials ⚠️
flighthandlers_client_13.go	54.95%	40 Missing and 10 partials ⚠️
flight_13.go	14.28%	36 Missing ⚠️
handshaker_13.go	58.46%	21 Missing and 6 partials ⚠️
flighthandler_13.go	42.85%	8 Missing ⚠️
util.go	68.18%	7 Missing ⚠️
pkg/protocol/handshake/handshake.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #829      +/-   ##
==========================================
- Coverage   82.44%   79.72%   -2.73%     
==========================================
  Files         121      128       +7     
  Lines        6928     7527     +599     
==========================================
+ Hits         5712     6001     +289     
- Misses        805     1064     +259     
- Partials      411      462      +51     

Flag	Coverage Δ
go	79.72% <46.92%> (-2.73%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.