Add feature to allow Aura collator to use full PoV size (#5393)

This PR introduces a feature that allows to optionally enable using the
full PoV size.

Technically, we're ready to enable it by default, but as corresponding
runtime changes have not been propagated to the system parachain
runtimes yet, doing so could put them at risk. On the other hand, there
are teams that could benefit from it right now, and it makes no sense
for them to wait for the fellowship release and everything.

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Co-authored-by: Andrei Sandu <[email protected]>