Add support for storing a source HRN in BOLT 12 `invoice_request`s

Add support for storing a source HRN in BOLT 12 `invoice_request`s

When we resolve a Human Readable Name to a BOLT 12 `offer`, we may
end up resolving to a wildcard DNS name covering all possible
`user` parts. In that case, if we just blindly pay the `offer`, the
recipient would have no way to tell which `user` we paid.

Instead, BOLT 12 defines a field to include the HRN resolved in the
`invoice_request`, which we implement here.

We also take this opportunity to remove constant parameters from
the `outbound_payment.rs` interface to `channelmanager.rs`