If an Offer contains a path, the blinded_node_id of the path's final hop
can be used as the signing pubkey. Make Offer::signing_pubkey and
OfferContents::signing_pubkey return an Option to support this. Upcoming
commits will implement this behavior.

If an offer has at least one path, it may omit the signing pubkey and
use the blinded node id of the last hop of a path to sign an invoice.
Allow parsing such offers but not yet creating them.

Instead of reusing OfferTlvStream::paths, add a dedicated paths TLV to
InvoiceRequestTlvStream such that it can be used in Refund. This allows
for an Offer without a signing_pubkey and still be able to differentiate
whether an invoice is for an offer or a refund.

When parsing a Bolt12Invoice use both the Offer's signing_pubkey and
paths to determine if it is for an Offer or a Refund. Previously, an
Offer was required to have a signing_pubkey. But now that it is
optional, the Offers paths can be used to make the determination.
Additionally, check that the invoice matches one of the blinded node ids
from the paths' last hops.

Offers currently require a description, though this may change to be
optional. Remove the description requirement from the API, setting and
empty string by default.

Refunds currently require a description, though this may change to be
optional. Remove the description requirement from the API, setting and
empty string by default.

The spec was changed to allow excluding an offer description if the
offer doesn't have an amount. However, it is still required when the
amount is set.