This PR makes it so we route the `UserService::get_users_by_ids` call
through Cloud instead of hitting the database.

We've introduced a new `CloudUserService` that will fetch the users from
Cloud using the internal API. Note that we've only implemented the
`get_users_by_ids` method on this service, as the endpoints for the
other methods don't yet exist.

We have also introduced a `TransitionalUserService` for the purposes of
gradually transitioning these calls over to Cloud. Right now it uses the
`CloudUserService` for the `get_users_by_ids` implementation, but then
uses the `DatabaseUserService` for the other methods.

Closes CLO-740.

Release Notes:

- N/A