Improve SignedExtension matching logic and remove SkipCheckIfFeeless bits (#1283)

* First pass making matching on signed exts more general and handlng SkipCheckifFeeless

* remove unneeded derives (only exts we can decode into are handled by the user)

* No SkipCheckIfFeeless in integration tests either

* Cargo fmt

* Remove SkipCheckIfFeeless specific logic

* clippy

* matches to just return bool, not result

* remove now-invalid comment

* return error from find if .iter() errors

subxt/src/tx/tx_client.rs

@@ -124,8 +124,7 @@ impl<T: Config, C: OfflineClientT<T>> TxClient<T, C> {
             account_nonce,
             self.client.clone(),
             other_params,
-        )
-        .map_err(Into::into)?;
+        )?;
 
         // Return these details, ready to construct a signed extrinsic from.
         Ok(PartialExtrinsic {