Add roles to add-group-member API

[seanh]

See also: #9141

Auth clients can add members to groups by calling the add-member-to-group API. Currently this API only allows members to be added with the roles ["member"]. Add a new, optional roles parameter to the request's JSON body (default: ["member"]) that allows auth clients to set the role in the same request that creates the membership.

Don't forget to update the API documentation at https://h.readthedocs.io/en/latest/api-reference/v1/#tag/groups/paths/~1groups~1%7Bid%7D~1members~1%7Buser%7D/post.