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 75.43860% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.99%. Comparing base (93c2677) to head (13d3194).
⚠️ Report is 1 commits behind head on master.

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #793      +/-   ##
==========================================
- Coverage   82.07%   81.99%   -0.09%     
==========================================
  Files         111      113       +2     
  Lines        6501     6558      +57     
==========================================
+ Hits         5336     5377      +41     
- Misses        764      773       +9     
- Partials      401      408       +7     

Flag	Coverage Δ
go	81.99% <75.43%> (-0.09%)	⬇️

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