Add support for GSSKerberosCredential in ElytronSubjectFactory

The JCA ElytronSubjectFactory must be improved to handle GSSKerberosCredentials to enable kerberos based authentication for databases.

It should use a CredentialCallback of type GSSKerberosCredential to obtain the GSSCredential and associated KerberosTicket when building the Subject that is used to establish the connections to the DB. If the authentication configuration matched by the configured authentication context references a kerberos security factory, this factory will be used to create the credential and ticket that will in turn be inserted into the constructed Subject instance.