Payment Links missing link to related payments

[janpaepke]

Somewhat ironically the payment links are missing the links to the payments made for a payment link.

For both Customers, as well as Subscriptions the API allows a user to retrieve all associated payments, similar to payment links.
For both these resources the returned _links object contains a payments object referencing the resource URL to retrieve corresponding payments.

Unfortunately for payment links (at least according to the docs) it does not.
I believe this should be implemented for consistency reasons.

For a more in-depth analysis, see #416

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Edit:
I found two more resources, which allow to retrieve associated payments, also returning a payments object in their _links response, strengthening the consistency argument:

• Settlements
• Profile

[janpaepke]

@hreinberger looking at the docs for Customers and Subscriptions now, I can't see any payment object in the returned _links prop. Was this removed? Or does it exist and is only missing from the docs?

What's the plan here?