feat(admin-ui): Add support for "payment method relation" custom field. Add "payment method data" service

[Stefanyshyn]

Description

I have added a component to display the "payment method relation" custom field.
I also added the "payment method data" service. Part of the related logic has been moved from the "settings data" service to the new service.

Breaking changes

This PR doesn't include any breaking changes. It only extends the existing functionality

Screenshots

You can see what the "payment method relation" custom field looks like in the video below

payment-method-relation-custom-field.mp4

Checklist

📌 Always:

• I have set a clear title
• My PR is small and contains a single feature
• I have checked my own PR

⚡ Most of the time:

• I have added or updated test cases
• I have updated the README if needed

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[michaelbromley]

Hi!

Thanks for this - it looks like it would be useful to ship in the admin ui. 2 points:

• this is a new feature so should go in the minor branch.
• the refactor you did with the data service could actually be a breaking change if someone is relying on dataService.settings.createPaymentMethod() etc. I would prefer you leave that part alone and just include the new component.