Support paying BOLT12 offers

[ ] add Bolt12 variant to the input parser. Make it return at least all the necessary fields to be able to call fetchinvoice.
[ ] add fetchinvoice to NodeAPI
[ ] Make a send_payment flow for bolt12. It calls fetchinvoice with the supplied parameters (sane defaults) and then pays the returned invoice by calling pay. Note that the current node_api.send_payment function decodes the invoice, so that's not compatible right now.

breez / breez-sdk-greenlight