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.
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