collator protocol changes for elastic scaling (validator side) (#3302)

Fixes #3128.

This introduces a new variant for the collation response from the
collator that includes the parent head data. For now, collators won't
send this new variant. We'll need to change the collator side of the
collator protocol to detect all the cores assigned to a para and send
the parent head data in the case when it's more than 1 core.

- [x] validate approach
- [x] check head data hash