Use a custom content entity type to store the request

Problem/Motivation

There are some advantages of using a dedicated entity:

We don't create the consumer entity from the beginning but only when the request is accepted
We have a storage for user who requested the client (which may be different from the user that owns the client)
We can store the request & rejection reasons
We can keep a history of requests and even of rejections.
It's possible to store de desired label from the very beginning