This adds the security `_authenticate` API to the HLREST client.
It is unlike some of the other APIs because the request does not
have a body.
The commit also creates the `User` entity. It is important
to note that the `User` entity does not have the `enabled`
flag. The `enabled` flag is part of the response, alongside
the `User` entity.
Moreover this adds the `SecurityIT` test class
(extending `ESRestHighLevelClientTestCase`).
Relates #29827
Albert Zaharovits