Add business logic preventing proving .unlock() without a signature from the recipient.

[maht0rz]

In case where the HTLC is being used for custom tokens, the actual funds transfer is done outside of the logic of the HTLC contract.
This means the the contract itself isn't in control of the balance update, since it's done in the same transaction, but not within the circuit. As a result of this, we have to add additional authorisation that prevents a valid proof from being created unless the caller can provide a signature of the intended fund recipient.

This issue is referenced as TODO here:
https://github.com/stove-labs/zk-htlc/blob/develop/src/HTLCPoseidon.ts#L200