Allow setting payment_id in send and send_using_amount in bolt11 send.

[G8XSU]

This is required to ensure idempotency in send_payment requests.

[tnull]

I don't think want to allow users to set their own payment ID as we derive it differently for different payment types and use it as an index in the payment store. I'm not quite sure how this is connected to idempotency? Could you expand in which context this came up?