headers: validate CWT claims

[pranjalkole]

We could also refactor this to a validateCWTClaims function in cwt.go

[SteveLasker]

@veraison/go-cose-maintainers, can we get some 👀's on this?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 3.63636% with 53 lines in your changes missing coverage. Please review.

Project coverage is 89.18%. Comparing base (eb9cdec) to head (c88a9da). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
headers.go	3.63%	53 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
- Coverage   91.12%   89.18%   -1.95%     
==========================================
  Files          12       12              
  Lines        2006     2053      +47     
==========================================
+ Hits         1828     1831       +3     
- Misses        122      168      +46     
+ Partials       56       54       -2     

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[SteveLasker]

@shizhMSFT, @qmuntal, @thomas-fossati can you PTAL?

[SteveLasker]

@pranjalkole can you please take a look at above changes?