Add a condition for payment types to only appear in the admin area or per order state

Problem/Motivation

We're using "Manual" payment method type only for admins to allow correction of payments in the admin area:
/admin/commerce/orders/12345/payments/add

Still there's not a perfectly matching condition in the payment gateway form to mark a payment type not to be available in checkout, but only in the admin area (at /admin/commerce/config/payment-gateways/manage/manual)

We're currently solving this by using user roles, but for these, of course, the payment method then also shows up in the checkout.

Steps to reproduce

Proposed resolution

Add a condition to control if this payment method should only show up in the admin area and not in checkout.
Maybe there's even a more clever condition?

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet