Move {min,max}_payment_size_msat into opening fee params

[JssDWt]

In LSPS2, the fields min_payment_size_msat and max_payment_size_msat are part of the get_info response. BitcoinAndLightningLayerSpecs/lsp#82 moves them into the opening_fee_params.

We'll still have to support opening_fee_params that do not contain these fields. The only place for that as far as I can tell is the paramsHash function in the opening service. It can try to validate with those fields first and then without as well to ensure backward compatibility.