Implement RBAC layer for `runtimeconfig` package

Related to https://github.com/coder/coder/issues/14586

If a cache layer is introduced, then `dbauthz` would be skipped for cache hits. Since the `dbauthz` is a general settings update, the `dbauthz` layer cannot sufficiently distinguish the rbac properties of a setting by it's `key string` alone.

RBAC needs to be woven into `runtimeconfig`.

One idea is to add some rbac element to `Initialize` and `New` so that each field knows it's own RBAC properties.