kid in COSEKey should be a bstr

[gsiou]

Hi,

Opened an issue on the dependency too jfromaniello/cose-kit#5, but opening here too since mdl is probably one of the most common cose-kit users.

According to https://www.iana.org/assignments/cose/cose.xhtml#key-common-parameters, kid should be a byte string. The dependency cose-kit seems to produce a tstr instead, as seen on the examples and unit tests. This affects COSE Keys embedded in the mdl, like the deviceKeyInfo in IssuerAuth.

What are your thoughts on this?

Thanks!