Implement the methods `getAllAssignments()`, `getAllRoles`, `getAllRules()` in `Manager::class`

[terabytesoftw]

I think it would be a good thing to return these methods in manager since they are resolved within the class, which is ideal for creating an rbac service.

[samdark]

It's good to have these but it won't work for real-world cases:

1. If there are too many assignments (and that is very likely) pagination is needed.
2. For a good UI, more it needed: you'll likely need an ability to search and auto-complete items etc.

[samdark]

I think it's a good idea to release without these and then add it in a subsequent major release.