Improve convenience of user and ACLs usage

[Yamakaky]

Is your feature request related to a problem? Please describe.
Currently, it seems that the only way to create redis users and associated ACLs declaratively is via the acl.secret.secretName directive in the cluster CRD. The associated secret contains a list of users and their permissions. To give access to a pod to its credentials, one either needs to manually create a secret with only the user's credential, or give it access to the main acl secret which defeats the purpose of having multiple users and access rights.

Describe the solution you'd like
I think something similar to what rabbitmq operator does would be appropriate. It allows the definition of users via CRD. Credentials are automatically generated and stored in a secret. Then, rabbitmq is configured with all the defined users and permissions. https://www.rabbitmq.com/kubernetes/operator/using-topology-operator.html#users-permissions.

Describe alternatives you've considered
I'm not sure what would be the best way to generate the current ACL file setup. I guess a script that generate the ACL + creates a secret per user? It would work but would be way less convenient.

What version of redis-operator are you using?

redis-operator version: 0.15

Additional context