pkg/swap: allow passing custom preimage to GetInvoice

[rolznz]

For GetInvoice it generates its own preimage which cannot be accessed from the outside. It would be great if a custom preimage can be passed in (or the submarineSwap function it calls to be made public so we can call it with a custom preimage)

[rolznz]

Alternatively, even just returning the preimage in the response of reverseSwap would be good