paras: Add runtime events for PVF pre-checking (#4683)

In this PR, paras module emit runtime events on certain PVF pre-checking
related conditions.

Specifically, there are 3 new events in the paras module:

1. PvfCheckStarted
2. PvfCheckAccepted
3. PvfCheckRejected

All of those have identifiers for the parachain that triggered the PVF
pre-checking and the validation code that goes through the pre-checking.

The mechanics of those are as follows. Each time a new PVF is added, be
it due to onboarding or upgrading, the `PvfCheckStarted` will be
triggered. If another parachain triggers a pre-checking process for the
validation code which is already being pre-checked, another
`PvfCheckStarted` event will be triggered with the corresponding para
id.

When the PVF pre-checking voting for a PVF was finished, several
`PvfCheckAccepted/Rejected` events will be triggered: one for each para id that
was subscribed to this check (i.e. was a "cause" for it).

If the PVF pre-checking is disabled, then one can still expect these
events to be fired. Since insta PVF approval is syncronous, the
`PvfCheckStarted` will be followed by the `PvfCheckAccepted` with the
same validation code and para id.

If somebody is interested in following validation code changes for a PVF
of a parachain, they would need to subscribe to those events. I did not
supply the topics for the events, since I am not sure if that's needed
or will be used, but they can be added later if needed.

polkadot/runtime/parachains/src/paras/mod.rs

@@ -314,6 +314,16 @@ enum PvfCheckCause<BlockNumber> {
 	},
 }
 
+impl<BlockNumber> PvfCheckCause<BlockNumber> {
+	/// Returns the ID of the para that initiated or subscribed to the pre-checking vote.
+	fn para_id(&self) -> ParaId {
+		match *self {
+			PvfCheckCause::Onboarding(id) => id,
+			PvfCheckCause::Upgrade { id, .. } => id,
+		}
+	}
+}
+
 /// Specifies what was the outcome of a PVF pre-checking vote.
 #[derive(Copy, Clone, Encode, Decode, RuntimeDebug, TypeInfo)]
 enum PvfCheckOutcome {
@@ -467,6 +477,15 @@ pub mod pallet {
 		NewHeadNoted(ParaId),
 		/// A para has been queued to execute pending actions. `para_id`
 		ActionQueued(ParaId, SessionIndex),
+		/// The given para either initiated or subscribed to a PVF check for the given validation
+		/// code. `code_hash` `para_id`
+		PvfCheckStarted(ValidationCodeHash, ParaId),
+		/// The given validation code was rejected by the PVF pre-checking vote.
+		/// `code_hash` `para_id`
+		PvfCheckAccepted(ValidationCodeHash, ParaId),
+		/// The given validation code was accepted by the PVF pre-checking vote.
+		/// `code_hash` `para_id`
+		PvfCheckRejected(ValidationCodeHash, ParaId),
 	}
 
 	#[pallet::error]
@@ -1345,6 +1364,9 @@ impl<T: Config> Pallet<T> {
 	) -> Weight {
 		let mut weight = 0;
 		for cause in causes {
+			weight += T::DbWeight::get().reads_writes(3, 2);
+			Self::deposit_event(Event::PvfCheckAccepted(*code_hash, cause.para_id()));
+
 			match cause {
 				PvfCheckCause::Onboarding(id) => {
 					weight += Self::proceed_with_onboarding(*id, sessions_observed);
@@ -1432,6 +1454,9 @@ impl<T: Config> Pallet<T> {
 			// Now we need to unbump it.
 			weight += Self::decrease_code_ref(code_hash);
 
+			weight += T::DbWeight::get().reads_writes(3, 2);
+			Self::deposit_event(Event::PvfCheckRejected(*code_hash, cause.para_id()));
+
 			match cause {
 				PvfCheckCause::Onboarding(id) => {
 					// Here we need to undo everything that was done during `schedule_para_initialize`.
@@ -1713,6 +1738,9 @@ impl<T: Config> Pallet<T> {
 	) -> Weight {
 		let mut weight = 0;
 
+		weight += T::DbWeight::get().reads_writes(3, 2);
+		Self::deposit_event(Event::PvfCheckStarted(code_hash, cause.para_id()));
+
 		weight += T::DbWeight::get().reads(1);
 		match PvfActiveVoteMap::<T>::get(&code_hash) {
 			None => {