Expose Offer/InvoiceRequest methods in Invoice

Expose Offer/InvoiceRequest methods in Invoice

Bolt12Invoice can either be for an Offer (via an InvoiceRequest) or a
Refund. It wraps those types, so expose their methods on both
Bolt12Invoice and UnsignedBolt12Invoice.

Since Refund does not have all the Offer/InvoiceRequest methods, use an
Option return type such that None can returned for refund-based
invoices.

For methods that are duplicated between Offer/InvoiceRequest and
Bolt12Invoice, prefer the (non-Option, if applicable) method from
Bolt12Invoice (e.g., amount_msats, signing_pubkey).