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>

Cargo.lock

@@ -5505,6 +5505,7 @@ dependencies = [
  "proc-macro-warning",
  "proc-macro2",
  "quote",
+ "regex",
  "sp-core-hashing",
  "syn 2.0.39",
 ]
@@ -6879,6 +6880,7 @@ dependencies = [
  "pallet-election-provider-multi-phase",
  "pallet-election-provider-support-benchmarking",
  "pallet-elections-phragmen",
+ "pallet-example-tasks",
  "pallet-fast-unstake",
  "pallet-glutton",
  "pallet-grandpa",
@@ -9842,6 +9844,22 @@ dependencies = [
  "sp-std 8.0.0",
 ]
 
+[[package]]
+name = "pallet-example-tasks"
+version = "1.0.0-dev"
+dependencies = [
+ "frame-benchmarking",
+ "frame-support",
+ "frame-system",
+ "log",
+ "parity-scale-codec",
+ "scale-info",
+ "sp-core",
+ "sp-io",
+ "sp-runtime",
+ "sp-std 8.0.0",
+]
+
 [[package]]
 name = "pallet-examples"
 version = "4.0.0-dev"
@@ -9853,6 +9871,7 @@ dependencies = [
  "pallet-example-kitchensink",
  "pallet-example-offchain-worker",
  "pallet-example-split",
+ "pallet-example-tasks",
 ]
 
 [[package]]