Add a `RecipientOnionFields` argument to spontaneous payment sends

Add a `RecipientOnionFields` argument to spontaneous payment sends

While most lightning nodes don't (currently) support providing a
payment secret or payment metadata for spontaneous payments,
there's no specific technical reason why we shouldn't support
sending those fields to a recipient.

Further, when we eventually move to allowing custom TLV entries in
the recipient's onion TLV stream, we'll want to support it for
spontaneous payments as well.

Here we simply add the new `RecipientOnionFields` struct as an
argument to the spontaneous payment send methods. We don't yet
plumb it through the payment sending logic, which will come when we
plumb the new struct through the sending logic to replace the
existing payment secret arguments.