Filter for requesting membership to a room

When a user wants to enter a room, they now have to request membership (see: https://gitlab.com/particify/dev/common-concerns/-/issues/3).

This requires the HTTP GW to provide a filter for when membership is requested, to chain a request to the auth service, adding membership.

See https://gitlab.com/particify/dev/foss/arsnova-backend/-/issues/110 for details on route, request and response body.