Because offers are a very generic mechanism, handling them can require interacting with an inventory system (do we actually have the quantity that the payer is requesting) or other such systems which do not have their place inside eclair. For this reason offer handlers must be implemented as plugins that communicate with the offer manager.
On startup, the offer handlers must register their offers with the offer manager, the offer manager will then forward the invoice requests and blinded payments to the relevant offer handler for approval.

- remove unnecessary `Try`
- rename `GetIncomingPaymentActor` commands
- restore standard -> blinded failure check

…s on OnionMessages.scala

Dummy is usually used for test or invalid data, whereas here we're just
storing a minimal version of the Bolt 12 invoice, so we rename it
MinimalBolt12Invoice.

We also add an explicit constructor for it that forces callers to provide
the data we expect to store.

Note that we remove the `features` field: using `nodeParams.features`
could be incorrect if the invoice_request's features don't include some
of our node's optional features. This is unused now anyway, but more
future-proof this way.

This commit refactors the offer manager without any meaningful change
in the business logic itself:

- move types to a separate file to isolate codec details
- make plugin data optional
- add debug logs in failure cases
- let plugin specify the `invoice_error` message
- avoid repeated calls to `Behaviors.setup`

- split OfferManager tests
- add more tests to the OfferManager
- remove test duplication in MultiPartHandlerSpec
- refactor BlindedPaymentSpec