This change adds documentation for the SAML APIs in Elasticsearch
and adds simple instructions on how these APIs can be used to
authenticate a user with SAML by a custom web application other
than Kibana.
Resolves: #40352
The change replaces the Vagrant box based fixture with a fixture
based on docker compose and 2 docker images, one for an openldap
server and one for a Shibboleth SAML Identity Provider.
The configuration of both openldap and shibboleth is identical to
the previous one, in order to minimize required changes in the
tests