Allow kas redis configuration to use `cache` redis configuration

Summary

Currently as seen at https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/charts/gitlab/charts/kas/templates/_helpers.tpl#L21-22 currently kas is hardcoded to look for and use configuration from the sharedState redis cluster. We would like to have the option to specify a different redis cluster (like cache) and have the settings pulled from that configuration instead.

(Please provide the steps to reproduce the issue)

(Please provide a sanitized version of the configuration used wrapped in a code block (```yaml))

(Paste sanitized configuration here)

Only sharedState Redis server can be used, as is statically configured

User is able to configure which of the Redis shards to make use of.

(Please provide any relevate log snippets you have collected, using code blocks (```) to format)

Edited Mar 04, 2021 by Jason Plum