Tasks: general system for recognizing and executing service work (#1343)

`polkadot-sdk` version of original tasks PR located here:
https://github.com/paritytech/substrate/pull/14329

Fixes #206

## Status
- [x] Generic `Task` trait
- [x] `RuntimeTask` aggregated enum, compatible with
`construct_runtime!`
- [x] Casting between `Task` and `RuntimeTask` without needing `dyn` or
`Box`
- [x] Tasks Example pallet
- [x] Runtime tests for Tasks example pallet
- [x] Parsing for task-related macros
- [x] Retrofit parsing to make macros optional
- [x] Expansion for task-related macros
- [x] Adds support for args in tasks
- [x] Retrofit tasks example pallet to use macros instead of manual
syntax
- [x] Weights
- [x] Cleanup
- [x] UI tests
- [x] Docs

## Target Syntax
Adapted from
https://github.com/paritytech/polkadot-sdk/issues/206#issue-1865172283

```rust
// NOTE: this enum is optional and is auto-generated by the other macros if not present
#[pallet::task]
pub enum Task<T: Config> {
    AddNumberIntoTotal {
        i: u32,
    }
}

/// Some running total.
#[pallet::storage]
pub(super) type Total<T: Config<I>, I: 'static = ()> =
StorageValue<_, (u32, u32), ValueQuery>;

/// Numbers to be added into the total.
#[pallet::storage]
pub(super) type Numbers<T: Config<I>, I: 'static = ()> =
StorageMap<_, Twox64Concat, u32, u32, OptionQuery>;

#[pallet::tasks_experimental]
impl<T: Config<I>, I: 'static> Pallet<T, I> {
	/// Add a pair of numbers into the totals and remove them.
	#[pallet::task_list(Numbers::<T, I>::iter_keys())]
	#[pallet::task_condition(|i| Numbers::<T, I>::contains_key(i))]
	#[pallet::task_index(0)]
	pub fn add_number_into_total(i: u32) -> DispatchResult {
		let v = Numbers::<T, I>::take(i).ok_or(Error::<T, I>::NotFound)?;
		Total::<T, I>::mutate(|(total_keys, total_values)| {
			*total_keys += i;
			*total_values += v;
		});
		Ok(())
	}
}
```

---------

Co-authored-by: Nikhil Gupta <17176722+gupnik@users.noreply.github.com>
Co-authored-by: kianenigma <kian@parity.io>
Co-authored-by: Nikhil Gupta <>
Co-authored-by: Gavin Wood <gavin@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: gupnik <nikhilgupta.iitk@gmail.com>

substrate/bin/node/runtime/Cargo.toml

@@ -80,6 +80,7 @@ pallet-democracy = { path = "../../../frame/democracy", default-features = false
 pallet-election-provider-multi-phase = { path = "../../../frame/election-provider-multi-phase", default-features = false }
 pallet-election-provider-support-benchmarking = { path = "../../../frame/election-provider-support/benchmarking", default-features = false, optional = true }
 pallet-elections-phragmen = { path = "../../../frame/elections-phragmen", default-features = false }
+pallet-example-tasks = { path = "../../../frame/examples/tasks", default-features = false }
 pallet-fast-unstake = { path = "../../../frame/fast-unstake", default-features = false }
 pallet-nis = { path = "../../../frame/nis", default-features = false }
 pallet-grandpa = { path = "../../../frame/grandpa", default-features = false }
@@ -177,6 +178,7 @@ std = [
 	"pallet-election-provider-multi-phase/std",
 	"pallet-election-provider-support-benchmarking?/std",
 	"pallet-elections-phragmen/std",
+	"pallet-example-tasks/std",
 	"pallet-fast-unstake/std",
 	"pallet-glutton/std",
 	"pallet-grandpa/std",
@@ -279,6 +281,7 @@ runtime-benchmarks = [
 	"pallet-election-provider-multi-phase/runtime-benchmarks",
 	"pallet-election-provider-support-benchmarking/runtime-benchmarks",
 	"pallet-elections-phragmen/runtime-benchmarks",
+	"pallet-example-tasks/runtime-benchmarks",
 	"pallet-fast-unstake/runtime-benchmarks",
 	"pallet-glutton/runtime-benchmarks",
 	"pallet-grandpa/runtime-benchmarks",
@@ -353,6 +356,7 @@ try-runtime = [
 	"pallet-democracy/try-runtime",
 	"pallet-election-provider-multi-phase/try-runtime",
 	"pallet-elections-phragmen/try-runtime",
+	"pallet-example-tasks/try-runtime",
 	"pallet-fast-unstake/try-runtime",
 	"pallet-glutton/try-runtime",
 	"pallet-grandpa/try-runtime",