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/support/src/lib.rs

@@ -2465,6 +2465,9 @@ pub mod pallet_macros {
 	/// Finally, the `RuntimeTask` can then used by a script or off-chain worker to create and
 	/// submit such tasks via an extrinsic defined in `frame_system` called `do_task`.
 	///
+	/// When submitted as unsigned transactions (for example via an off-chain workder), note
+	/// that the tasks will be executed in a random order.
+	///
 	/// ## Example
 	#[doc = docify::embed!("src/tests/tasks.rs", tasks_example)]
 	/// Now, this can be executed as follows:

substrate/frame/support/src/traits/tasks.rs

@@ -46,6 +46,10 @@ pub trait Task: Sized + FullCodec + TypeInfo + Clone + Debug + PartialEq + Eq {
 	fn iter() -> Self::Enumeration;
 
 	/// Checks if a particular instance of this `Task` variant is a valid piece of work.
+	///
+	/// This is used to validate tasks for unsigned execution. Hence, it MUST be cheap
+	/// with minimal to no storage reads. Else, it can make the blockchain vulnerable
+	/// to DoS attacks.
 	fn is_valid(&self) -> bool;
 
 	/// Performs the work for this particular `Task` variant.