In upcoming commits, we need to check whether a static invoice or its underlying
offer is expired in no-std builds. Here we expose the methods to do so. The
methods could instead be kept private to the crate, but they seem potentially
useful.

As part of serving static invoices to payers on behalf of often-offline
recipients, these recipients need a way to contact the static invoice server to
retrieve blinded paths to include in their offers.

Add a utility to create blinded paths for this purpose as a static invoice
server. The recipient will be configured with the resulting paths and use them
to request offer paths on startup.

As part of serving static invoices to payers on behalf of often-offline
recipients, we need to provide the async recipient with blinded message paths
to include in their offers.

Support responding to inbound requests for offer paths from async recipients.

As part of serving static invoices to payers on behalf of often-offline
recipients, the recipient will send us the final static invoice once it's done
being interactively built. We will then persist this invoice and confirm to
them that the corresponding offer is ready to be used for async payments.

Surface an event once the invoice is received and expose an API to tell the
recipient that it's ready for payments.

Here we implement serving static invoices to payers on behalf of often-offline
recipients. These recipients previously encoded blinded paths terminating at
our node in their offer, so we receive invoice requests on their behalf.

Handle those inbound invreqs by retrieving a static invoice we previously
persisted on behalf of the payee, and forward it to the payer as a reply to
their invreq.

We're about to add a bunch more async payments tests, so take this opportunity
to clean up the existing tests by no longer hardcoding the keysend payment
preimage bytes ahead of time. This previously caused an MPP test to spuriously
fail because all the session_privs were the same, and is generally not ideal.

Also add a few comments to an existing test and a few more trivial cleanups.

We were manually creating the static invoice in tests, but now we can use the
static invoice server protocol to interactively build the invoice.