Adds syntax for marking calls feeless (#1926)

Fixes https://github.com/paritytech/polkadot-sdk/issues/1725

This PR adds the following changes:
1. An attribute `pallet::feeless_if` that can be optionally attached to
a call like so:
```rust
#[pallet::feeless_if(|_origin: &OriginFor<T>, something: &u32| -> bool {
	*something == 0
})]
pub fn do_something(origin: OriginFor<T>, something: u32) -> DispatchResult {
     ....
}
```
The closure passed accepts references to arguments as specified in the
call fn. It returns a boolean that denotes the conditions required for
this call to be "feeless".

2. A signed extension `SkipCheckIfFeeless<T: SignedExtension>` that
wraps a transaction payment processor such as
`pallet_transaction_payment::ChargeTransactionPayment`. It checks for
all calls annotated with `pallet::feeless_if` to see if the conditions
are met. If so, the wrapped signed extension is not called, essentially
making the call feeless.

In order to use this, you can simply replace your existing signed
extension that manages transaction payment like so:
```diff
- pallet_transaction_payment::ChargeTransactionPayment<Runtime>,
+ pallet_skip_feeless_payment::SkipCheckIfFeeless<
+	Runtime,
+	pallet_transaction_payment::ChargeTransactionPayment<Runtime>,
+ >,
```

### Todo
- [x] Tests
- [x] Docs
- [x] Prdoc

---------

Co-authored-by: Nikhil Gupta <>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

substrate/bin/node/runtime/Cargo.toml

@@ -129,6 +129,7 @@ pallet-transaction-payment = { path = "../../../frame/transaction-payment", defa
 pallet-transaction-payment-rpc-runtime-api = { path = "../../../frame/transaction-payment/rpc/runtime-api", default-features = false}
 pallet-asset-conversion-tx-payment = { path = "../../../frame/transaction-payment/asset-conversion-tx-payment", default-features = false}
 pallet-asset-tx-payment = { path = "../../../frame/transaction-payment/asset-tx-payment", default-features = false}
+pallet-skip-feeless-payment = { path = "../../../frame/transaction-payment/skip-feeless-payment", default-features = false}
 pallet-transaction-storage = { path = "../../../frame/transaction-storage", default-features = false}
 pallet-uniques = { path = "../../../frame/uniques", default-features = false}
 pallet-vesting = { path = "../../../frame/vesting", default-features = false}
@@ -212,6 +213,7 @@ std = [
 	"pallet-scheduler/std",
 	"pallet-session-benchmarking?/std",
 	"pallet-session/std",
+	"pallet-skip-feeless-payment/std",
 	"pallet-society/std",
 	"pallet-staking-runtime-api/std",
 	"pallet-staking/std",
@@ -308,6 +310,7 @@ runtime-benchmarks = [
 	"pallet-salary/runtime-benchmarks",
 	"pallet-scheduler/runtime-benchmarks",
 	"pallet-session-benchmarking/runtime-benchmarks",
+	"pallet-skip-feeless-payment/runtime-benchmarks",
 	"pallet-society/runtime-benchmarks",
 	"pallet-staking/runtime-benchmarks",
 	"pallet-state-trie-migration/runtime-benchmarks",
@@ -381,6 +384,7 @@ try-runtime = [
 	"pallet-salary/try-runtime",
 	"pallet-scheduler/try-runtime",
 	"pallet-session/try-runtime",
+	"pallet-skip-feeless-payment/try-runtime",
 	"pallet-society/try-runtime",
 	"pallet-staking/try-runtime",
 	"pallet-state-trie-migration/try-runtime",

substrate/bin/node/runtime/src/lib.rs

@@ -569,6 +569,10 @@ impl pallet_asset_conversion_tx_payment::Config for Runtime {
 		pallet_asset_conversion_tx_payment::AssetConversionAdapter<Balances, AssetConversion>;
 }
 
+impl pallet_skip_feeless_payment::Config for Runtime {
+	type RuntimeEvent = RuntimeEvent;
+}
+
 parameter_types! {
 	pub const MinimumPeriod: Moment = SLOT_DURATION / 2;
 }
@@ -1394,7 +1398,11 @@ where
 			frame_system::CheckEra::<Runtime>::from(era),
 			frame_system::CheckNonce::<Runtime>::from(nonce),
 			frame_system::CheckWeight::<Runtime>::new(),
-			pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::<Runtime>::from(tip, None),
+			pallet_skip_feeless_payment::SkipCheckIfFeeless::from(
+				pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::<Runtime>::from(
+					tip, None,
+				),
+			),
 		);
 		let raw_payload = SignedPayload::new(call, extra)
 			.map_err(|e| {
@@ -2134,6 +2142,7 @@ construct_runtime!(
 		Statement: pallet_statement,
 		Broker: pallet_broker,
 		Mixnet: pallet_mixnet,
+		SkipFeelessPayment: pallet_skip_feeless_payment,
 	}
 );
 
@@ -2160,7 +2169,10 @@ pub type SignedExtra = (
 	frame_system::CheckEra<Runtime>,
 	frame_system::CheckNonce<Runtime>,
 	frame_system::CheckWeight<Runtime>,
-	pallet_asset_conversion_tx_payment::ChargeAssetTxPayment<Runtime>,
+	pallet_skip_feeless_payment::SkipCheckIfFeeless<
+		Runtime,
+		pallet_asset_conversion_tx_payment::ChargeAssetTxPayment<Runtime>,
+	>,
 );
 
 /// Unchecked extrinsic type as expected by this runtime.