Support LDAP role mapping of SASL EXTERNAL credentials

currently the textcertificate login module must be used with SASL EXTERNAL. There is no other way to do authorisation and role assignment.
however a validated TLS certificate subject dn is a valid identity, in the same way as a kerberos token identity. If we provide a login module that will populate a subject principal with the subject DN, it will be possible to chain with the LDAPLoginModule and have LDAP used for role assignment. In LDAP, the CERT subjectDN just needs to be added as a member to any existing role definition.
LDAPLoginModule can be configured to not authenticate, not lookup the user and just do role assignment.

authenticateUser=false and default/empty userSearchMatching