Create new `InvoiceFeatures` object for Invoice-specific features

Create new `InvoiceFeatures` object for Invoice-specific features

In the past we skipped doing this since invoice parsing occurs in a
different crate. However, we need to accept InvoiceFeatures in routing
now that we support MPP route collection, to detect if we can select
multiple paths or not. Further, we should probably take
rust-lightning-invoice as either a module or a subcrate in this repo.