This PR improves internationalized domain name handling according
to rules of IDNA and based on the description in RFC 5280, section 7:
https://datatracker.ietf.org/doc/html/rfc5280#section-7.
Support for internationalized URI(s), so-called IRIs, still needs to
be done.
A nebula provisioner will generate a X509 or SSH certificate with
the identities in the nebula certificate embedded in the token.
The token is signed with the private key of the nebula certificate.