Support including invreqs when building payment onions

Support including invreqs when building payment onions

Add a new invoice request parameter to onion_utils::create_payment_onion. As of
this commit it will always be passed in as None, to be updated in future
commits.

Per BOLTs PR 1149, when paying a static invoice we need to include our original
invoice request in the HTLC onion since the recipient wouldn't have received it
previously.