Add public key and certificate root list to the eIDAS Link

[iamtxena]

For future compatibility without certificates, it should be necessary to store the public key independently of the certificate.
For the current compatibility with certificates, it should be necessary to store also the certificate root list.
Both items should be passed as arguments for the eidas_link_did API function. certificate and certificate_list are OPTIONAL.
So, the call should be:

def eidas_link_did(did, pub_key, proof, padding = PSS_PADDING, certificate = None, certificate_root_list = None) -> str