Add a `transfer` (for single recipient)

Should support specifying the total amount, and the fee comes out of this total amount.
We could do `transfer` (single recipient, with a `deduct_fee` boolean) and [`multi_transfer`](https://github.com/Benjamin-Loison/Internet-Computer-Bitcoin-Library/blob/d4e6ba70c1baa5e17524f0af1c2ca03e3694bc28/rust/src/agent.rs#L230--L302) (multiple recipients possible, no boolean).