Allow adding a custom field to the PaymentMethod, ShippingMethod, CustomerGroup entities

[nemo-js]

It would be very helpful, if we could create a custom field for PaymentMethods and ShippingMethods.

The use-case we want to implement is the following:
At the front-end we should display a different UI depending on the payment method (ie on cash just proceed, on Stripe redirect to Stripe etc)
Another use-case is that we want to hide the address UI when the Order is store-pickup

[Draykee]

I also think that CustomerGroup might be a good candidate to get extended with customFields aswell.
Right now it just has a name (https://www.vendure.io/docs/typescript-api/entities/customer-group/) but maybe: description, notes, group limit, links to social groups (like a facebook group?), stats, or other CRM properties might be helpful.

[sepehrg]

I need custom fields for customerGroup too. Thank you for your efforts.

[michaelbromley]

I'll be adding custom field support on:

• Country
• CustomerGroup
• PaymentMethod
• Promotion
• TaxCategory
• TaxRate
• Zone

(ShippingMethod is actually already supported)