Read redis connection information from existing secret

[aidan-aisy]

Similar to postgres, I'd like to be able to specify an existing secret for redis credentials as an alternative to hard coding them.

[mitchnielsen]

Thanks for reaching out @aidan-aisy. This sounds like a good follow-up to #523 which seems to only allow the credentials to come from individual Helm values.

To leave some breadcrumbs: we'd likely want to adjust this helper to support getting the the value from an existing Secret, similar to the code a few lines above that's used when the built-in Redis is enabled.