Introduce storage attr macro #[disable_try_decode_storage] and set it on System::Events and ParachainSystem::HostConfiguration (#3454)

Closes https://github.com/paritytech/polkadot-sdk/issues/2560

Allows marking storage items with `#[disable_try_decode_storage]`, and
uses it with `System::Events`.

Question: what's the recommended way to write a test for this? I
couldn't find a test for similar existing macro `#[whitelist_storage]`.

cumulus/pallets/parachain-system/src/lib.rs

@@ -840,6 +840,7 @@ pub mod pallet {
 	///
 	/// This data is also absent from the genesis.
 	#[pallet::storage]
+	#[pallet::disable_try_decode_storage]
 	#[pallet::getter(fn host_configuration)]
 	pub(super) type HostConfiguration<T: Config> = StorageValue<_, AbridgedHostConfiguration>;