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.
This commit adds documentation for configuring Kerberos realm.
Configuring Kerberos realm documentation highlights important
terminology and requirements before creating Kerberos realm.
Most of the documentation is centered around configuration from
Elasticsearch rather than go deep into Kerberos implementation.
Kerberos realm settings are mentioned in the security settings
for Kerberos realm.
