Pass relay parent as argument when collating (#1789)

This pr changes the collation function to also pass the current relay
parent the parachain block should be build on.

polkadot/roadmap/implementers-guide/src/node/collators/collation-generation.md

@@ -36,7 +36,10 @@ pub struct Collation {
 
 struct CollationGenerationConfig {
   key: CollatorPair,
-  collator: Box<dyn Fn(&ValidationData) -> Box<dyn Future<Output = Option<Collation>>>>
+  /// Collate will be called with the relay chain hash the parachain should build 
+  /// a block on and the `ValidationData` that provides information about the state
+  /// of the parachain on the relay chain.
+  collator: Box<dyn Fn(Hash, &ValidationData) -> Box<dyn Future<Output = Option<Collation>>>>
   para_id: ParaId,
 }
 ```