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/frame/support/test/tests/construct_runtime_ui/generics_in_invalid_module.stderr

@@ -1,4 +1,4 @@
-error: `Call` is not allowed to have generics. Only the following pallets are allowed to have generics: `Event`, `Error`, `Origin`, `Config`.
+error: `Call` is not allowed to have generics. Only the following pallets are allowed to have generics: `Event`, `Error`, `Origin`, `Config`, `Task`.
   --> tests/construct_runtime_ui/generics_in_invalid_module.rs:24:36
    |
 24 |         Balance: balances::<Instance1>::{Call<T>, Origin<T>},

substrate/frame/support/test/tests/construct_runtime_ui/invalid_module_details_keyword.stderr

@@ -1,4 +1,4 @@
-error: expected one of: `Pallet`, `Call`, `Storage`, `Event`, `Error`, `Config`, `Origin`, `Inherent`, `ValidateUnsigned`, `FreezeReason`, `HoldReason`, `LockId`, `SlashReason`
+error: expected one of: `Pallet`, `Call`, `Storage`, `Event`, `Error`, `Config`, `Origin`, `Inherent`, `ValidateUnsigned`, `FreezeReason`, `HoldReason`, `Task`, `LockId`, `SlashReason`
   --> tests/construct_runtime_ui/invalid_module_details_keyword.rs:23:20
    |
 23 |         system: System::{enum},

substrate/frame/support/test/tests/construct_runtime_ui/invalid_module_entry.stderr

@@ -1,4 +1,4 @@
-error: expected one of: `Pallet`, `Call`, `Storage`, `Event`, `Error`, `Config`, `Origin`, `Inherent`, `ValidateUnsigned`, `FreezeReason`, `HoldReason`, `LockId`, `SlashReason`
+error: expected one of: `Pallet`, `Call`, `Storage`, `Event`, `Error`, `Config`, `Origin`, `Inherent`, `ValidateUnsigned`, `FreezeReason`, `HoldReason`, `Task`, `LockId`, `SlashReason`
   --> tests/construct_runtime_ui/invalid_module_entry.rs:24:23
    |
 24 |         Balance: balances::{Unexpected},

substrate/frame/support/test/tests/construct_runtime_ui/number_of_pallets_exceeds_tuple_size.stderr

@@ -76,3 +76,11 @@ help: consider importing one of these items
    |
 18 + use frame_support::traits::PalletInfo;
    |
+
+error[E0412]: cannot find type `RuntimeTask` in this scope
+  --> tests/construct_runtime_ui/number_of_pallets_exceeds_tuple_size.rs:39:1
+   |
+39 | #[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: you might have meant to use the associated type: `Self::RuntimeTask`
+   |
+   = note: this error originates in the macro `frame_system::config_preludes::TestDefaultConfig` which comes from the expansion of the macro `frame_support::macro_magic::forward_tokens_verbatim` (in Nightly builds, run with -Z macro-backtrace for more info)