[FC-0099] fix: return user count of the specific scope

[BryanttV]

Description

This PR fixes the current behavior of the "list role by scope" endpoint. Previously, the user count corresponded to the total number of users in the scope, that is, the total number of users assigned to all libraries. Now, it only returns the total number of users assigned to the specific library provided in the scope query parameter.

Testing Instructions

Using Tutor:

1. Install this plugin with the changes in this PR.
2. Run the migrations by running tutor dev exec lms python manage.py lms migrate
3. Load the default policies by running tutor dev exec lms python manage.py lms load_policies
4. Assign different roles to users in different scopes (different libraries).

Endpoint

GET http://local.openedx.io:8000/api/authz/v1/roles/?scope={your-scope}'

It should only return the count of users assigned to each role within the specified scope.

Merge Checklist:

Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[mariajgrimaldi]

I've also had to do this a couple of times in these functions since get_filtered_grouping_policy doesn't support filtering by more than one parameter. It'd be nice if, as a future improvement, we could implement a get_filtered_grouping_policy with N parameters as input. In any case, this looks good to me!

[bmtcril]

Looks good to me, though I'm not sure if there is a use case for keeping the original around too. Should be easy enough to resurrect if we need to, though.