Introduce new Runtime API endpoint for fetching the validation data (#3728)

* Introduce new Runtime API endpoint

`persisted_validation_data_with_code_hash` that will be used
by the candidate validation subsystem in order to decrease amount
of runtime API requests.

* Node-side part of new runtime API request

* Define code hash getter via macro

* Rename new endpoint to `assumed_validation_data`

* Docs for runtime API impl of new endpoint

* AssumedValidationData specialized request function

* fmt

polkadot/runtime/parachains/src/paras.rs

@@ -365,6 +365,7 @@ pub mod pallet {
 	///
 	/// Corresponding code can be retrieved with [`CodeByHash`].
 	#[pallet::storage]
+	#[pallet::getter(fn current_code_hash)]
 	pub(super) type CurrentCodeHash<T: Config> =
 		StorageMap<_, Twox64Concat, ParaId, ValidationCodeHash>;
 
@@ -623,7 +624,7 @@ impl<T: Config> Pallet<T> {
 
 	/// The validation code of live para.
 	pub(crate) fn current_code(para_id: &ParaId) -> Option<ValidationCode> {
-		CurrentCodeHash::<T>::get(para_id).and_then(|code_hash| {
+		Self::current_code_hash(para_id).and_then(|code_hash| {
 			let code = CodeByHash::<T>::get(&code_hash);
 			if code.is_none() {
 				log::error!(