collator-side: send parent head data (#3521)

On top of #3302.

We want the validators to upgrade first before we add changes to the
collation side to send the new variants, which is why this part is
extracted into a separate PR.

The detection of when to send the parent head is based on the core
assignments at the relay parent of the candidate. We probably want to
make it more flexible in the future, but for now, it will work for a
simple use case when a para always has multiple cores assigned to it.

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Signed-off-by: Matteo Muraca <mmuraca247@gmail.com>
Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: Matteo Muraca <56828990+muraca@users.noreply.github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Juan Ignacio Rios <54085674+JuaniRios@users.noreply.github.com>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>