Consolidate API names related to Key

[qmuntal]

This PR contain the following changes:

• Renamed NewXKey to NewKeyX, being X the key type. This form is more idiomatic.
• Removed the Key prefix from Key properties. It is go-ish to avoid prefixing the name of a property with the name of the struct.
• Rename XInvalid to XReserved, being X KeyType, Curve, KeyOp and Algorithm. This makes our API more similar to the COSE spec naming.
• Added some tests and spec pointers.

[codecov[bot]]

Codecov Report

Merging #161 (adad1c2) into main (a5dc571) will increase coverage by 0.20%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #161      +/-   ##
==========================================
+ Coverage   93.59%   93.80%   +0.20%     
==========================================
  Files          11       11              
  Lines        1687     1695       +8     
==========================================
+ Hits         1579     1590      +11     
+ Misses         75       72       -3     
  Partials       33       33              

Impacted Files	Coverage Δ
algorithm.go	100.00% <100.00%> (+14.63%)	:arrow_up:
headers.go	92.19% <100.00%> (-0.91%)	:arrow_down:
key.go	95.94% <100.00%> (+0.03%)	:arrow_up:

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[qmuntal]

PTAL @thomas-fossati @shizhMSFT

[yogeshbdeshpande]

PTAL @thomas-fossati @shizhMSFT

@qmuntal : Thomas is on a long vacation. Please get the necessary comments from @shizhMSFT and then proceed with submission.