Replace `PaymentSecret` with `RecipientOnionFields` in the pub API

Replace `PaymentSecret` with `RecipientOnionFields` in the pub API

This moves the public payment sending API from passing an explicit
`PaymentSecret` to a new `RecipientOnionFields` struct (which
currently only contains the `PaymentSecret`). This gives us
substantial additional flexibility as we look at add both
`PaymentMetadata`, a new (well, year-or-two-old) BOLT11 invoice
extension to provide additional data sent to the recipient.

In the future, we should also add the ability to add custom TLV
entries in the `RecipientOnionFields` struct.