Runtime API for historical validation code (#1893)

* fix: ensure candidate validation gets code based on occupied core assumption

* guide: runtime API for historical validation code

* add historical runtime API

* integrate into runtime API subsystem

* remove blocked TODO

* fix service build: enable notifications protocol only under real overseer

* Update node/subsystem/src/messages.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* fix compilation

Co-authored-by: Robert Habermeier <robert@Roberts-MacBook-Pro.local>
Co-authored-by: Sergei Shulepov <sergei@parity.io>

polkadot/roadmap/implementers-guide/src/runtime-api/historical_validation_code.md

@@ -0,0 +1,7 @@
+# Historical Validation Code
+
+Fetch the historical validation code used by a para for candidates executed in the context of a given block height in the current chain.
+
+```rust
+fn historical_validation_code(at: Block, para_id: ParaId, context_height: BlockNumber) -> Option<ValidationCode>;
+```