Add DTLS 1.3 Unified Header

[theodorsm]

Description

This PR implements the new DTLS 1.3 Unified Header that is part of the new record layer encoding.

https://datatracker.ietf.org/doc/html/rfc9147#name-the-dtls-record-layer:

    0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |0|0|1|C|S|L|E E|
    +-+-+-+-+-+-+-+-+
    | Connection ID |   Legend:
    | (if any,      |
    /  length as    /   C   - Connection ID (CID) present
    |  negotiated)  |   S   - Sequence number length
    +-+-+-+-+-+-+-+-+   L   - Length present
    |  8 or 16 bit  |   E   - Epoch
    |Sequence Number|
    +-+-+-+-+-+-+-+-+
    | 16 bit Length |
    | (if present)  |
    +-+-+-+-+-+-+-+-+

Reference issue

Partly fixes #755

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.31%. Comparing base (fd27a52) to head (00ca5b2).

Files with missing lines	Patch %	Lines
pkg/protocol/recordlayer/header_13.go	74.54%	7 Missing and 7 partials ⚠️
pkg/protocol/recordlayer/header.go	52.38%	7 Missing and 3 partials ⚠️

❌ Your patch check has failed because the patch coverage (69.23%) 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     #793      +/-   ##
==========================================
- Coverage   82.44%   82.31%   -0.14%     
==========================================
  Files         121      123       +2     
  Lines        6928     7005      +77     
==========================================
+ Hits         5712     5766      +54     
- Misses        805      819      +14     
- Partials      411      420       +9     

Flag	Coverage Δ
go	82.31% <69.23%> (-0.14%)	⬇️

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[theodorsm]

Hmm, we should also check if we have enabled DTLS 1.3 before parsing and returning something that would be unvalid for DTLS 1.2. Tied to TODOs in #738