Draft: Complement system.set_user_groups with get_user_groups

The "Add an API for setting the groups a user is a member of" e3dfb399 commit introduced the system.set_user_groups endpoint with no way to retrieve the current list of groups.

This commit complements it with an endpoint to list the current groups a user is member of.

Since the 2023.03 upstream release LAVA also ships a different and more complete users/groups API, so this commit is not meant to be upstreamed and it is just a stopgap until 2023.03 is used on production and the authz-mediator can start relying on the new APIs: https://git.lavasoftware.org/lava/lava/-/merge_requests/2066

This arguably may not belong to the collabora repo at all and should only be carried by downstreams actually using the authz-mediator, but since this was the easiest way to test things for me I am also pushing it here just in case.