Add Payjoin Receiver Support

[Camillarhi]

This PR adds support for receiving payjoin payments in LDK Node. This is currently a work in progress and implements the receiver side of the payjoin protocol.

• Add session store for persisting payjoin receiver events across restarts
• Implement KVStorePayjoinReceiverPersister to handle session persistence
• Add Payjoin as a PaymentKind to the payment store
• Add event polling mechanism for active payjoin sessions
• Wire up payjoin payment request and receive flows

Note on persistence: The payjoin library currently only supports synchronous persistence, but they're working on adding async support(payjoin/rust-payjoin#1235). This PR sets up the persistence structure (KVStorePayjoinReceiverPersister), which will be updated to use async operations once the upstream PR is merged.

This PR will partially address #177

[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!