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>