Add WeightToFee and LengthToFee impls to transaction-payment Runtime API (#13110)

* Add WeightToFee and LengthToFee impls to RPC

* Remove RPC additions

* Update frame/transaction-payment/rpc/runtime-api/src/lib.rs

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Add comments to length_to_fee and weight_to_fee

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>