Ignore optional extensions instead of erroring

[kamilwaz]

This PR updates the parser to ignore optional extensions instead of throwing an error.

I introduced a new error code invalid-extensions (aka server-error-value-ext from the RFC) that is returned when an extension is invalid. extensions-not-supported is still reserved for the m extension.

RFC:

Mandatory extensions sent by one peer but not understood by the
other MUST cause authentication failure (the server SHOULD send
the "extensions-not-supported" server-error-value).

Unknown optional extensions MUST be ignored upon receipt.¹

Footnotes

1. https://datatracker.ietf.org/doc/html/rfc5802#section-5.1 ↩

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.26%. Comparing base (10461b4) to head (b21dda7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   92.18%   92.26%   +0.07%     
==========================================
  Files           5        5              
  Lines         320      323       +3     
==========================================
+ Hits          295      298       +3     
  Misses         25       25              

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[NelsonVides]

Beautiful. I wrote all of this when I was reading like, my first ever RFC, missed that one! Thanks for catching it! 😄