Add OutgoingPaymentsDb interface

[t-bast]

We split payments into two parts: a top-level payment description and a list of partial payments (corresponding to the actual HTLCs that are sent). This allows storing them in different tables to avoid duplicating data. It also makes it easier in the future to implement different payment types (swaps, keysend, etc).

Notes:

• I'd like reviewers to focus mostly on the PaymentsDb file: the rest is implementation details
• we will need to figure out the serialization of these objects in the DB: it can either be done entirely in phoenix-kmm with SQL, but if that helps we can implement binary serialization of some fields (for example errors) in eclair-kmp, let met know

[t-bast]

I've made many changes to the IncomingPaymentsDb interface (based on our offline discussions), I think this result is really nice and future-proof now.

[dpad85]

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