Cannot create custom string labels for protected headers

[hmueller314]

Currently rfc8152 defines generic header parameters:
Generic_Headers = (
? 1 => int / tstr, ; algorithm identifier
? 2 => [+label], ; criticality
? 3 => tstr / int, ; content type
? 4 => bstr, ; key identifier
? 5 => bstr, ; IV
? 6 => bstr, ; Partial IV
? 7 => COSE_Signature / [+COSE_Signature] ; Counter signature
)
with labels in the range [1, 7]. The code maps string labels in the JSON files to those numeric values (e.g. "alg" -> 1), but some applications require custom string labels to be included in a protected headers. For example c2pa requires X.509 certificates to be stored in a header named "x5chain". This is currently not supported in COSE-C.