Add payment_hash support and improve HODL payment handling

[free-free-6]

HODL Invoice Call Flow

C (Merchant) -- BTC Invoice --> A (Consumer/RGB User) -- swap request --> B (Gateway) -- BTC --> C

1. POST /lninvoice — Create HODL invoice with payment_hash (extracted from C's BTC invoice). HTLC will be held, not auto-claimed.
2. A pays B's RGB invoice — PaymentClaimable event fires, payment stays Pending (no preimage).
3. POST /sendpayment — B pays C's original BTC invoice. C claims it, exposing preimage.
4. Auto-claim — PaymentSent event detects matching pending inbound payment, automatically calls claim_funds(preimage).

[zoedberg]

This doesn't seem to introduce/improve HODL invoices. HODL invoices are invoices that are not auto-claimed. This PR is just adding the possibility to decide the invoice payment hash, which makes sense only in the context of HODL invoices.
We already have a PR actually adding support to HODL invoices (#91), so this PR is not needed.