Remove unnecessary cbor marshal methods

[qmuntal]

This PR removes a bunch of methods added in #146 which, in my opinion, shouldn't have been added.

This is the list and the reasoning for removing them:

• Algorithm.UnmarshalCBOR: its sole purpose is to allow unmarshaling algorithms encoded as strings. We don't support any algorithm than can be references as a string, so this method always fail on such case. I would rather not add a code path for a thing that we don't support, just in case in the future we need to. It makes the code base bigger and more difficult to maintain.
• Algorithm.MarshalCBOR: it was only there as companion method for Algorithm.UnmarshalCBOR.
• KeyType.UnmarshalCBOR: same reasoning as in Algorithm.UnmarshalCBOR. Additionally, we should not error when the value is not supported by go-cose, as users might want to unmarshal a key using go-cose but create a signer/verifier using external code.
• Algorithm.MarshalCBOR: it was only there as companion method for KeyType.UnmarshalCBOR.
• Curve.MarshalCBOR: same reasoning as in KeyType.UnmarshalCBOR, also we shouldn't validate its value.
• Curve.MarshalCBOR: it was only there as companion method for Curve.UnmarshalCBOR.

Also, I've removed KeyOpt.IsSupported and its usage in KeyOpt.UnmarshalCBOR. RFC 7517 Section 4.3 says that other values may be used.

[codecov[bot]]

Codecov Report

Merging #155 (fec382b) into main (cebef14) will decrease coverage by 0.25%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
- Coverage   91.48%   91.24%   -0.25%     
==========================================
  Files          12       12              
  Lines        1633     1542      -91     
==========================================
- Hits         1494     1407      -87     
+ Misses        101       98       -3     
+ Partials       38       37       -1     

Impacted Files	Coverage Δ
algorithm.go	85.36% <ø> (-3.35%)	:arrow_down:
headers.go	93.09% <100.00%> (-0.13%)	:arrow_down:
key.go	88.86% <100.00%> (-0.85%)	:arrow_down:

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