Add support for redis sentinel

[emilioah]

Quick changes to support redis sentinel.

Configuration in moodle/config.php:

$CFG->auth_saml2_store = '\auth_saml2\redis_store';
// OPT 1: Single redis server
$CFG->auth_saml2_redis_server = '{{ moodle_config.redishost }}'; # Required for the redis_store above
// OPT 2: Redis Sentinel
$CFG->auth_saml2_redissentinel_servers = '1.1.1.1:26397,2.2.2.2:26397,3.3.3.3:26397';
$CFG->auth_saml2_redissentinel_group = 'rediscluster';

[danmarsden]

I know the existing code pollutes the global $CFG to store auth_saml2 specific settings, but we should really migrate those to config_plugins table instead if possible, although we might not block merging based on that for now... I'll see if someone here can PR this properly - thanks!

[matt-catalyst]

SimpleSAML has builtin support for Redis Sentinel - could it be utilised by this patch?

https://simplesamlphp.org/docs/stable/simplesamlphp-maintenance.html#configuring-redis-storage

[brendanheywood]

These really need to be plugin scope config items not globals please, its a very simple change. Get the config using get_config and set it in config.php like:

$CFG->forced_plugin_settings['auth_saml2']['foobar'] = 0;

This also needs to be documented somewhere like the readme file