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/frame/transaction-payment/skip-feeless-payment/src/mock.rs

@@ -0,0 +1,92 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use super::*;
+use crate as pallet_skip_feeless_payment;
+
+use frame_support::{derive_impl, parameter_types};
+use frame_system as system;
+
+type Block = frame_system::mocking::MockBlock<Runtime>;
+type AccountId = u64;
+
+#[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig)]
+impl frame_system::Config for Runtime {
+	type Block = Block;
+}
+
+impl Config for Runtime {
+	type RuntimeEvent = RuntimeEvent;
+}
+
+parameter_types! {
+	pub static PreDispatchCount: u32 = 0;
+}
+
+#[derive(Clone, Eq, PartialEq, Debug, Encode, Decode, TypeInfo)]
+pub struct DummyExtension;
+
+impl SignedExtension for DummyExtension {
+	type AccountId = AccountId;
+	type Call = RuntimeCall;
+	type AdditionalSigned = ();
+	type Pre = ();
+	const IDENTIFIER: &'static str = "DummyExtension";
+	fn additional_signed(&self) -> sp_std::result::Result<(), TransactionValidityError> {
+		Ok(())
+	}
+	fn pre_dispatch(
+		self,
+		_who: &Self::AccountId,
+		_call: &Self::Call,
+		_info: &DispatchInfoOf<Self::Call>,
+		_len: usize,
+	) -> Result<Self::Pre, TransactionValidityError> {
+		PreDispatchCount::mutate(|c| *c += 1);
+		Ok(())
+	}
+}
+
+#[frame_support::pallet(dev_mode)]
+pub mod pallet_dummy {
+	use frame_support::pallet_prelude::*;
+	use frame_system::pallet_prelude::*;
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(_);
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {
+		#[pallet::feeless_if(|_origin: &OriginFor<T>, data: &u32| -> bool {
+			*data == 0
+		})]
+		pub fn aux(_origin: OriginFor<T>, #[pallet::compact] _data: u32) -> DispatchResult {
+			unreachable!()
+		}
+	}
+}
+
+impl pallet_dummy::Config for Runtime {}
+
+frame_support::construct_runtime!(
+	pub struct Runtime {
+		System: system,
+		SkipFeeless: pallet_skip_feeless_payment,
+		DummyPallet: pallet_dummy,
+	}
+);