Fix extrinsic params for contract chains
Created by: ascjones
Fixes the SignedExtra
type to align with substrate-contracts-node
and the parachain runtime.
This PR changes it to PolkadotExtrinsicParams
which is just an alias to BaseExtrinsicParams<T, PlainTip>
which maps to the runtime SignedExtra1 definition with
pallet_transaction_payment::ChargeTransactionPayment`
Fixes #515 (closed)