runtime-api: add validation_code_hash API (#4629)

This is the first step to close
https://github.com/paritytech/polkadot/issues/4524

polkadot/roadmap/implementers-guide/src/runtime-api/validation-code.md

@@ -11,3 +11,11 @@ Fetch the validation code (past, present or future) by its hash.
 ```rust
 fn validation_code_by_hash(at: Block, ValidationCodeHash) -> Option<ValidationCode>;
 ```
+
+Fetch the validation code hash used by a para, making the given `OccupiedCoreAssumption`.
+
+> ⚠️ This API was introduced in `ParachainHost` v2.
+
+```rust
+fn validation_code_hash(at: Block, ParaId, OccupiedCoreAssumption) -> Option<ValidationCodeHash>;
+```