docs: Define api contract for User-Grouped role assignments

[rodmgwgu]

Closes: #405

The new Team Members tab design for the Admin Console requires fields that are not currently available in the API.

This ADR proposes the contract for updating the existing /api/authz/v1/users/ to support the new Team Members tab use cases.

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

AI Usage

Kiro was used to refine the prose and clarity of the text, help with rst formatting, as well as to assist on research.

The core ADR contents and direction were drafted by hand.

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[dcoa]

it is missing the roles filter

Suggested change

	`roles`` (optional): Comma-separated list of roles to filter by (e.g. ``course_auditor,library_admin``).

[dcoa]

the table shows the name of the scope instead of the id