Add "Requestable" boolean option per product (like Orderable, but for requests)

Problem/Motivation

In ✨ Add commerce_product_availibility_webform_request to allow webform order requests Fixed we added the request functionality, but currently it can't be enabled / disabled per product.

Seems I wasn't clear enough in my description in that task.

We shouldn't implement this until it's needed for a client or someone pays this feature.

Steps to reproduce

Proposed resolution

1. Add a checkbox above the base modules "Orderable" checkbox if the submodule is enabled with label "requestable"
2. Only show the request webform, if it's enabled to be requested

Remaining tasks

User interface changes

API changes

Data model changes