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

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Signed-off-by: Andrei Sandu <[email protected]>