Consolidate Payment Orders / Deposits / Checkouts into single "Payment Requests" workflow

[lullis]

The initial design led to many different ways to create payment requests.

• Deposits for hub users that want to top-up their account balance
• Payment Orders for hub users that want to request payment with specific amounts
• Checkouts for store systems that want to create an additional external payment identifier

We can and should consolidate all of these elements into a single "Payment Request" entity, that:

• DOES NOT require token amounts to be specified, and that can be considered paid upon the first confirmed payment
• has an OPTIONAL external identifier field