Adds ability to trigger tasks via unsigned transactions (#4075)

This PR updates the `validate_unsigned` hook for `frame_system` to allow
valid tasks to be submitted as unsigned transactions. It also updates
the task example to be able to submit such transactions via an off-chain
worker.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/frame/system/src/lib.rs

@@ -741,9 +741,7 @@ pub mod pallet {
 		#[cfg(feature = "experimental")]
 		#[pallet::call_index(8)]
 		#[pallet::weight(task.weight())]
-		pub fn do_task(origin: OriginFor<T>, task: T::RuntimeTask) -> DispatchResultWithPostInfo {
-			ensure_signed(origin)?;
-
+		pub fn do_task(_origin: OriginFor<T>, task: T::RuntimeTask) -> DispatchResultWithPostInfo {
 			if !task.is_valid() {
 				return Err(Error::<T>::InvalidTask.into())
 			}
@@ -1032,6 +1030,18 @@ pub mod pallet {
 					})
 				}
 			}
+			#[cfg(feature = "experimental")]
+			if let Call::do_task { ref task } = call {
+				if task.is_valid() {
+					return Ok(ValidTransaction {
+						priority: u64::max_value(),
+						requires: Vec::new(),
+						provides: vec![T::Hashing::hash_of(&task.encode()).as_ref().to_vec()],
+						longevity: TransactionLongevity::max_value(),
+						propagate: true,
+					})
+				}
+			}
 			Err(InvalidTransaction::Call.into())
 		}
 	}