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/support/procedural/src/lib.rs

@@ -1371,6 +1371,25 @@ pub fn whitelist_storage(_: TokenStream, _: TokenStream) -> TokenStream {
 	pallet_macro_stub()
 }
 
+/// The optional attribute `#[pallet::disable_try_decode_storage]` will declare the
+/// storage as whitelisted from decoding during try-runtime checks. This should only be
+/// attached to transient storage which cannot be migrated during runtime upgrades.
+///
+/// ### Example
+/// ```ignore
+/// 	#[pallet::storage]
+/// 	#[pallet::disable_try_decode_storage]
+/// 	pub(super) type Events<T: Config> = StorageValue<_, Vec<Box<EventRecord<T::RuntimeEvent, T::Hash>>>, ValueQuery>;
+/// ```
+///
+/// NOTE: As with all `pallet::*` attributes, this one _must_ be written as
+/// `#[pallet::disable_try_decode_storage]` and can only be placed inside a `pallet` module in order
+/// for it to work properly.
+#[proc_macro_attribute]
+pub fn disable_try_decode_storage(_: TokenStream, _: TokenStream) -> TokenStream {
+	pallet_macro_stub()
+}
+
 /// The `#[pallet::type_value]` attribute lets you define a struct implementing the `Get` trait
 /// to ease the use of storage types. This attribute is meant to be used alongside
 /// [`#[pallet::storage]`](`macro@storage`) to define a storage's default value. This attribute