Refactor Key to support custom parameters

[qmuntal]

This PR redesigns Key so all non-common parameters are stored in a map instead, even the ones from known key types (i.e. EC2, OKP, and Symmetric keys). This allows go-cose to support key types and parameters that are not defined in RFC8152 without having to break the module API. See #151 for more context.

The Key struct now looks like this:

type Key struct {
        // Common Params
    KeyType KeyType
    KeyID []byte
    Algorithm Algorithm
    KeyOps []KeyOp
    BaseIV []byte

        // Custom Params
    Params map[interface{}]interface{}
}

I've also added some helper functions to access the known and unknown key parameters:

// Useful to retrieve all parameters relevant for each known key type
func (*Key) OKP() (crv Curve, x []byte, d []byte)
func (*Key) EC2() (crv Curve, x []byte, y, d []byte)
func (*Key) Symmetric() (k []byte)

// Useful to retrieve individual parameters
func (*Key) ParamBytes(label interface{}) ([]byte, bool)
func (*Key) ParamInt(label interface{}) (int64, bool)
func (*Key) ParamUint(label interface{}) (uint64, bool)
func (*Key) ParamString(label interface{}) (string, bool)
func (*Key) ParamBool(label interface{}) (bool, bool)

A good amount of new test cases have been added.

@setrofim I've remove the intOrStr struct, as it is not necessary with the new way of decoding keys, It is a pity, because I conceptually liked it. If we ever release a new major version, we should consider using something like that to have a better type enforcement in all map labels.

[codecov[bot]]

Codecov Report

Merging #158 (0c37dca) into main (c862f87) will increase coverage by 2.14%. The diff coverage is 96.66%.

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   91.16%   93.30%   +2.14%     
==========================================
  Files          12       11       -1     
  Lines        1550     1658     +108     
==========================================
+ Hits         1413     1547     +134     
+ Misses        101       78      -23     
+ Partials       36       33       -3     

Impacted Files	Coverage Δ
key.go	95.76% <96.66%> (+6.46%)	:arrow_up:

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[qmuntal]

@thomas-fossati @setrofim I've added one last commit to this PR, in case you want to take another look.

[thomas-fossati]

@thomas-fossati @setrofim I've added one last commit to this PR, in case you want to take another look.

neat!