feat/ocw/bookkeeping (#5200)

Co-Authored-By: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

substrate/primitives/externalities/src/lib.rs

@@ -47,6 +47,9 @@ pub enum Error {
 ///
 /// Provides access to the storage and to other registered extensions.
 pub trait Externalities: ExtensionStore {
+	/// Write a key value pair to the offchain storage database.
+	fn set_offchain_storage(&mut self, key: &[u8], value: Option<&[u8]>);
+
 	/// Read runtime storage.
 	fn storage(&self, key: &[u8]) -> Option<Vec<u8>>;