Introduce CollectCollationInfo runtime api (#443)

* Introduce `CollectCollationInfo` runtime api

Instead of using well known keys to communicate information about a
collation between the runtime and the collator, we now use a runtime api
for this.

* Fixes bug

* Apply suggestions from code review

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

* Doc update

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

test/runtime/src/lib.rs

@@ -395,6 +395,12 @@ impl_runtime_apis! {
 			UpgradeDetection::get()
 		}
 	}
+
+	impl cumulus_primitives_core::CollectCollationInfo<Block> for Runtime {
+		fn collect_collation_info() -> cumulus_primitives_core::CollationInfo {
+			ParachainSystem::collect_collation_info()
+		}
+	}
 }
 
 cumulus_pallet_parachain_system::register_validate_block!(Runtime, Executive);