Query customizable roles for groups members page

What does this MR do and why?

To display customizable roles assigned to a member, we need to query the associated customizable roles. With static roles, a user has single well-defined role via inheritance. However, multiple customizable roles could be associated to the same user in the group hierarchy.

This is the backend part of the feature querying the data needed to be displayed in the members page.

Related to #420625 (closed).

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