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>

prdoc/pr_3795.prdoc

@@ -0,0 +1,14 @@
+title: Enable collators to build on multiple cores
+
+doc:
+  - audience: Node Dev
+    description: |
+      Introduces a `CoreIndex` parameter in `SubmitCollationParams`. This enables
+      the collators to make use of potentially multiple cores assigned at some relay
+      chain block. This extra parameter is used by the collator protocol and collation
+      generation subsystems to forward the collation to the approapriate backing group.
+
+crates:
+- name: polkadot-node-collation-generation
+- name: polkadot-collator-protocol
+  bump: minor