Authorization Realms allow an authenticating realm to delegate the task
of constructing a User object (with name, roles, etc) to one or more
other realms.

E.g. A client could authenticate using PKI, but then delegate to an LDAP
realm. The LDAP realm performs a "lookup" by principal, and then does
regular role-mapping from the discovered user.

This commit includes:
- authorization_realm support in the pki, ldap, saml & kerberos realms
- docs for authorization_realms
- checks that there are no "authorization chains"
   (whereby "realm-a" delegates to "realm-b", but "realm-b" delegates to "realm-c")

Authorization realms is a platinum feature.