Query if a user should exist on the application service.
GET {{baseUrl}}/users/:userId
This endpoint is invoked by the homeserver on an application service to query
the existence of a given user ID. The homeserver will only query user IDs
inside the application service's users
namespace. The homeserver will
send this request when it receives an event for an unknown user ID in
the application service's namespace, such as a room invite.
RESPONSES
status: OK
{}