collator protocol: always send full parent head data in collation response

[alindima]

We added the possibility of sending the full parent head data (not just the hash as it used to be) for elastic scaling.
This was done in #3521, only for paras that have multiple assigned cores.

However, having this always present would allow for some simplifications in the validator side of the collator protocol. We'd no longer need to keep candidates in

polkadot-sdk/polkadot/node/network/collator-protocol/src/validator_side/mod.rs

Line 408 in 7211692

blocked_from_seconding: HashMap<BlockedCollationId, Vec<PendingCollationFetch>>,

I propose we always send the full parent head data from the collator. This won't enable us to do the simplification I mentioned above rightaway. That can only be done once all collators upgrade

[MrishoLukamba]

on it @alindima @skunert
will take these related issues also #7731

[skunert]

Hey @MrishoLukamba , I think these collator-protocol issues are already in progress by @alindima.