Explore whether `PaidBolt12Invoice` should be moved to `PaymentMetadataStore`

[tnull]

In #733 we introduced a bolt12_invoice: Option<PaidBolt12Invoice> field to PaymentSuccessful.

While this works, it's a bit awkward to have a BOLT12-specific field on an event that is also used for other payment means. After #811 and #448 we should reconsider whether we should rather move storing the paid invoice to PaymentMetadata store, which might also allow to avoid the clone-on-persisting for the ffi version of the PaidBolt12Invoice.