Expose Custom-Hash JIT Invoice APIs #285

After reviewing the LSPS2 surface following #274, I noticed that the pinned ldk-node exposes receive_via_jit_channel_for_hash and receive_variable_amount_via_jit_channel_for_hash, while ldk-server only exposes the JIT methods that generate their own payment hashes.

Since ldk-server already supports non-JIT Bolt11ReceiveForHash together with manual claim/fail, would matching custom-hash JIT RPCs be welcome?