Centralize key validations and check more invalid cases

[qmuntal]

This PR does the following:

• Make Key.Validate private. The checks in that method are not exhaustive, e.g., we not validating that the key cryptographically ok. I would rather keep it private so we can evolve it without being tied to an specific behavior.
• Validate that all required key parameters exists depending on the key type.
• Key.PublicKey and Key.PrivateKey now call Key.validate so we don't return a malformed key.
• NewSymmetricKey no longer returns an error, as this function can't fail.
• Add more test cases

[codecov]

Codecov Report

Merging #156 (c768d2b) into main (2f778da) will increase coverage by 0.47%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
+ Coverage   91.05%   91.52%   +0.47%     
==========================================
  Files          12       12              
  Lines        1542     1545       +3     
==========================================
+ Hits         1404     1414      +10     
+ Misses        101       94       -7     
  Partials       37       37              

Impacted Files	Coverage Δ
key.go	89.88% <84.61%> (+1.01%)	⬆️

... and 1 file with indirect coverage changes

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[qmuntal]

@thomas-fossati @yogeshbdeshpande @shizhMSFT please take a look.

[shizhMSFT]

LGTM

[thomas-fossati]

awesome, thanks!