Support pallet::storage conditional compilation (#8324)

* Support pallet::storage conditional compilation.

* Add docs for cfg attributes.

* Keep strong types for get cfg attrs return.

* Update frame/support/procedural/src/pallet/parse/helper.rs

* Update frame/support/procedural/src/pallet/parse/storage.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/frame/support/src/lib.rs

@@ -1418,6 +1418,18 @@ pub mod pallet_prelude {
 /// pub(super) type MyStorage<T> = StorageMap<_, Blake2_128Concat, u32, u32>;
 /// ```
 ///
+/// The optional attributes `#[cfg(..)]` allow conditional compilation for the storage.
+///
+/// E.g:
+/// ```ignore
+/// #[cfg(feature = "my-feature")]
+/// #[pallet::storage]
+/// pub(super) type MyStorage<T> = StorageValue<_, u32>;
+/// ```
+///
+/// All the `cfg` attributes are automatically copied to the items generated for the storage, i.e. the
+/// getter, storage prefix, and the metadata element etc.
+///
 /// NOTE: If the `QueryKind` generic parameter is still generic at this stage or is using some type
 /// alias then the generation of the getter might fail. In this case the getter can be implemented
 /// manually.