collation-generation + collator-protocol: collate on multiple assigned cores (#3795)

This works only for collators that implement the `collator_fn` allowing
`collation-generation` subsystem to pull collations triggered on new
heads.

Also enables
`request_v2::CollationFetchingResponse::CollationWithParentHeadData` for
test adder/undying collators.

TODO:
- [x] fix tests
- [x] new tests
- [x] PR doc

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/node/subsystem-types/src/messages.rs

@@ -228,6 +228,8 @@ pub enum CollatorProtocolMessage {
 		/// The result sender should be informed when at least one parachain validator seconded the
 		/// collation. It is also completely okay to just drop the sender.
 		result_sender: Option<oneshot::Sender<CollationSecondedSignal>>,
+		/// The core index where the candidate should be backed.
+		core_index: CoreIndex,
 	},
 	/// Report a collator as having provided an invalid collation. This should lead to disconnect
 	/// and blacklist of the collator.