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]`.

substrate/frame/system/src/lib.rs

@@ -894,6 +894,7 @@ pub mod pallet {
 	/// just in case someone still reads them from within the runtime.
 	#[pallet::storage]
 	#[pallet::whitelist_storage]
+	#[pallet::disable_try_decode_storage]
 	#[pallet::unbounded]
 	pub(super) type Events<T: Config> =
 		StorageValue<_, Vec<Box<EventRecord<T::RuntimeEvent, T::Hash>>>, ValueQuery>;