Allow external signers to modify the payload (#6030)

* remove validateSignedTransaction

* add optional allowCallDataAlteration defaulting to true

* nit

* spacing

* Update packages/api/src/submittable/createClass.ts

packages/types/src/types/extrinsic.ts

@@ -188,6 +188,7 @@ export interface IExtrinsicEra extends Codec {
 }
 
 export interface SignatureOptions {
+  allowCallDataAlteration?: boolean;
   blockHash: Uint8Array | string;
   era?: IExtrinsicEra;
   genesisHash: Uint8Array | string;