Add an invoice/offer store

[Camillarhi]

Adds an invoice/offer store to track invoices separately from payments.

This PR introduces a dedicated invoice/offer store that:

• Stores created invoices independently of payment state
• Links invoices to their associated PaymentId(s)
• Allows querying invoices before payment events arrive

Closes #298

[ldk-reviews-bot]

👋 Hi! I see this is a draft PR.
I'll wait to assign reviewers until you mark it as ready for review.
Just convert it out of draft status when you're ready for review!

[Camillarhi]

This is in draft because as discussed elsewhere, it needs to be built on top of #425