MIssing `alg` header in the Mdoc device signing key

[antonio-ivanovski]

When verifying the mdoc device signature with the included device key, the key itself fail to import within the auth0/mdl library (internally using cose-kit to convert to JWK and import with jose). The issue is that the alg header is not included in the device signing key part of the mdoc.

More info can be fond at: auth0-lab/mdl#61
Creating the same issue here as it is probably good idea to include the alg header so the verifiers don't have to guess the value.

[antonio-ivanovski]

auth0-lab/mdl#64