Expose proper `Invoice` / `Offer` types in bindings

[tnull]

Previously, we'd expose Bolt11Invoice/Offer/Refunds as strings in bindings that get translated at time of use via UniffiCustomTypeConverters. This has a number of drawbacks, maybe most notably that these types don't allow the user to validate any fields before paying them.

We should expose 'propertypes for these (i.e., likely we'd need to create newtype wrappers for at least some of them based on theuniffi` feature flag) that allow retrieving all the important fields (amount, description, payer_note, etc).

[alexanderwiederin]

Happy to look into this!