This commit adds an OpenID Connect authentication realm to
elasticsearch. Elasticsearch (with the assistance of kibana or
another web component) acts as an OpenID Connect Relying
Party and supports the Authorization Code Grant and Implicit
flows as described in http://ela.st/oidc-spec. It adds support
for consuming and verifying signed ID Tokens, both RP
initiated and 3rd party initiated Single Sign on and RP
initiated signle logout.
It also adds an OpenID Connect Provider in the idp-fixture to
be used for the associated integration tests.
This is a backport of #40674
Ioannis Kakavas