Compatibility with Gin content form

I don't feel comfortable about this behavior in the context of a promotion form, otherwise all should be good.

As part of my MR I only added support to selected entities:
- order
- product
- product_variation
- promotion
- coupon
- store

Some notes:
- Not all content entities support sidebars (ie. coupon), which makes the form look a bit weird (very subjective opinion)
- For config entities we are better off without gin content form support as these forms are rather simple, featuring only handful of fields
- Some content entities are not to be edited in the context of admin UI (ie. payment_method) hence no need to support it either

Merged, thank you!