Goal
====

- manage the accounts of the http component via the MySQL Shell
- authenticate against accounts stored in the metadata-servers of InnoDB Cluster.
- using the REST APIs even when communication to the metadata-servers is lost.

Implementation
==============

Authentication is done based on the credentials from the 'router_rest_accounts'
table. Accounts and their privileges are cached on the router side at
runtime. Password hashes must be in the MCF format, only caching_sha2_password
passwords hashes are supported. Authorization is not supported, therefore
'privileges' field from the router_rest_accounts should be empty. Only
innodb-cluster metadata schema version 2.0.0 and higher are supported.

The http_auth_backend plugin is extended to support new 'metadata_cache' value for
the backend field.

Two new configuration options are added for the metadata_cache plugin:
- auth_cache_refresh_interval - time in seconds between auth-cache refresh
  attempts.
- auth_cache_ttl - time in seconds until the auth cache becomes invalid if not
  refreshed.
Both auth_cache_ttl and auth_cache_refresh_interval cannot be lett than the
'ttl' configuration option value. Additionaly auth_cache_refresh_interval
cannot be greater than auth_cache_ttl.

RB: 23537
Reviewed by: Andrzej Religa <andrzej.religa@oracle.com>
Approved by: Omar Mendez <omar.mendez@oracle.com>