Create user-facing RBAC roles

Hello!

I want to bring attention to this PR #396, it is required multi-tenant clusters where users limited to specific namespaces; bearer of `admin` role in such namespace have no rights to see any instances:
```
Error from server (Forbidden): dragonflies.dragonflydb.io is forbidden: User "oidc:***" cannot list resource "dragonflies"
in API group "dragonflydb.io" in the namespace "test-db"
```