[BUGFIX] Respect IRRE parent config in Ajax calls

The code to transfer the inline parent context to form engine
in Ajax requests exists but is currently non functional in some
situations.

The config is stored as array, which is hashed by serializing
the array, and building the hash on that string. However
that string is not transferred over the wire,
but the json encoded array.

If a float value was present at some place in this array,
json_encode and json_decode will add a slight offset
to these numbers than if the value is serialized.

To avoid such errors, the hmac is now calculated and
checked against the json encoded value.

We also clean up the code in this area to avoid duplication
and improve the hash calculation and comparison.

By doing so, we can clean up and simplify the flex form handling
for IRRE fields as well.

Resolves: #79999
Releases: master
Change-Id: I049d699f9f30edad0a9c8b06bbc3970e2cdac417
Reviewed-on: https://review.typo3.org/51783


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>