Add Cookie extension with unit test and fuzzing

[theodorsm]

Description

DTLS 1.3 has moved it's cookie handling to an extension.

Reference issue

Fixes #769

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.44%. Comparing base (713910a) to head (f14e0b2).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/protocol/extension/cookie.go	84.61%	2 Missing and 2 partials ⚠️
pkg/protocol/extension/extension.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #770      +/-   ##
==========================================
- Coverage   81.45%   81.44%   -0.02%     
==========================================
  Files         102      103       +1     
  Lines        5689     5717      +28     
==========================================
+ Hits         4634     4656      +22     
- Misses        679      682       +3     
- Partials      376      379       +3     

Flag	Coverage Δ
go	81.44% <78.57%> (-0.02%)	⬇️

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[JoTurk]

Should we make a branch for dtls 1.3? I'm just worried we might break something for dtls 1.2 or if we want to make a release for dtls 1.2. cherry-picking commits might be hard after few months.
I know we talked about this before but we merged few bug fixes and enhancements since then.
Up to you. I'm just suggesting :)

[Sean-Der]

I am excited/curious to see how we do 1.2 and 1.3 in the code base at the same time?

@theodorsm Do you think you will share code between them, or will be a clear line in the code base? Do you think the code base will end up in a state where we can't make a release (but have a urgent 1.2 bug?)

[JoTurk]

@Sean-Der theo also talked about it here #738

[theodorsm]

@Sean-Der, @JoTurk Adding extension parsers shouldn't affect DTLS 1.2. I think it makes sense to have them in the master branch already. For more involved stuff we have to re-visit #738 (let's have a discussion about it there). crypto/tls implements all extensions side by side for tls 1.2 and tls 1.3

[theodorsm]

Also, fuzzing might be a bit overkill for this simple extension, but I think all unmarshalling of raw bytes should be fuzzed and this sets an easy example.

[theodorsm]

@JoTurk, thanks for the feedback! Should be fixed now.

[JoTurk]

Thank you for your fast fixes.