[NEW] Support database level ACL

[hpatro]

The problem/use-case that the feature addresses

Extend ACL to include a way to restrict users to a specific database. This would make it possible to assign individual databases to a user, opening the possibiity for a single Valkey instance to be multi-tenant.

Description of the feature

Enable ACL syntax to define DB level permissions that only allows a user to connect/select a specific database.

Additional Information

There is a separate ongoing discussion on support multiple DBs in Valkey cluster mode #1319. If we plan to support multiple DBs feature in cluster mode, we should rather make the guardrails stronger around it.

Past discussion: redis/redis#8099