In this commit, we add a new `LookupInvoiceV2` method attached to the
invoice RPC sub-server. Compared to the existing version, this version
allows an invoice to be looked up by an invoice ref. This enables users
to query an AMP invoice based on a specific _set ID_, so they can get
the information related to the set of cumulative settles to that
invoices.
We also test that legacy keysend payments are promoted to AMP payments
on the receiver-sdie by asserting basic properties of the fields
returned via the rpc.
