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/procedural/src/pallet/parse/helper.rs

@@ -47,7 +47,7 @@ pub trait MutItemAttrs {
 }
 
 /// Take the first pallet attribute (e.g. attribute like `#[pallet..]`) and decode it to `Attr`
-pub fn take_first_item_attr<Attr>(item: &mut impl MutItemAttrs) -> syn::Result<Option<Attr>> where
+pub fn take_first_item_pallet_attr<Attr>(item: &mut impl MutItemAttrs) -> syn::Result<Option<Attr>> where
 	Attr: syn::parse::Parse,
 {
 	let attrs = if let Some(attrs) = item.mut_item_attrs() {
@@ -69,18 +69,29 @@ pub fn take_first_item_attr<Attr>(item: &mut impl MutItemAttrs) -> syn::Result<O
 }
 
 /// Take all the pallet attributes (e.g. attribute like `#[pallet..]`) and decode them to `Attr`
-pub fn take_item_attrs<Attr>(item: &mut impl MutItemAttrs) -> syn::Result<Vec<Attr>> where
+pub fn take_item_pallet_attrs<Attr>(item: &mut impl MutItemAttrs) -> syn::Result<Vec<Attr>> where
 	Attr: syn::parse::Parse,
 {
 	let mut pallet_attrs = Vec::new();
 
-	while let Some(attr) = take_first_item_attr(item)? {
+	while let Some(attr) = take_first_item_pallet_attr(item)? {
 		pallet_attrs.push(attr)
 	}
 
 	Ok(pallet_attrs)
 }
 
+/// Get all the cfg attributes (e.g. attribute like `#[cfg..]`) and decode them to `Attr`
+pub fn get_item_cfg_attrs(attrs: &[syn::Attribute]) -> Vec<syn::Attribute> {
+	attrs.iter().filter_map(|attr|  {
+		if attr.path.segments.first().map_or(false, |segment| segment.ident == "cfg") {
+			Some(attr.clone())
+		} else {
+			None
+		}
+	}).collect::<Vec<_>>()
+}
+
 impl MutItemAttrs for syn::Item {
 	fn mut_item_attrs(&mut self) -> Option<&mut Vec<syn::Attribute>> {
 		match self {