Rlecellier/request response serializers

[rlecellier]

Purpose

Improve backend endpoint typing.

[rlecellier]

--misstake--

[MorganeAD]

Hello !
In this commit I made a proposition of solution on Create Order route

I try to use your proposed request and response serializers for the creation of an Order.
All the cases keeps the same returned response, except the error on billing_address, now it return a dict of error and not a single string, because it's a field with multiple values.
All the old test past (modification of the billing_address error).
The generation of openAPI also works.
The usage of new request serializer add also checks on billing_address and credit_card_id values that was not done before.

I have also two questions on the existing code,

• if paiement fail because we didn't find the card, we didn't cancel the order ? (see commentary in the code)
• in joanie, address has a "title" field that is mandatory, does the front return this field when using this route to create an Order ?