Validate key sizes and allow signing with empty public keys

[qmuntal]

This PR adds the following features:

• Validate that the x, y and d parameters for OKP and EC2 keys have the right size.
• Recompute the x and y for OKP and EC in case they are omitted, which is allowed by the COSE spec.

[codecov[bot]]

Codecov Report

Merging #159 (fae3330) into main (bfe4717) will decrease coverage by 0.11%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #159      +/-   ##
==========================================
- Coverage   93.48%   93.38%   -0.11%     
==========================================
  Files          11       11              
  Lines        1658     1678      +20     
==========================================
+ Hits         1550     1567      +17     
- Misses         75       78       +3     
  Partials       33       33              

Impacted Files	Coverage Δ
key.go	95.90% <100.00%> (+0.13%)	:arrow_up:

... and 1 file with indirect coverage changes

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[OR13]

I looked at this, I agree its important to do, seems to be done correctly, but I am not a reliable CR for golang.

[SteveLasker]

@yogeshbdeshpande, @shizhMSFT can you PTAL