Support providing intermediate certificates for signing #64
Description
- mDL does not allow intermediate certs AFAIK, but for other types it is allowed
- it is not possible to provide it at the moment
- we should verify that when signing mDL doctype we do not allow intermediate
- we should verify that when verifying there are no intermediates (but this can be solved with Verify root certificate is valid IACA certificate according to ISO 18013-5 #63)