[BUGFIX] Ensure uncached forms in case of POST requests

A new `FormRequestHandler` has been introduced as entry point which
takes care of creating and rendering the actual `FormRuntime`.
Independent of the current application dispatcher (either the
default `FormFrontendController`, any custom Extbase controller
or a TypoScript `FLUIDTEMPLATE` cObject) any processing will
end up here.

To ensure proper caching, it is important to separate the two main
actions `render` and `perform`, which unfortunately cannot be resolved
easily, especially when using custom Extbase controller actions.
The term uncached refers to using cObject `USER_INT` instead of `USER`.

More details about the process flow are described in
`\TYPO3\CMS\Form\Core\FormRequestHandler`, introduced by this change.

Resolves: #93887
Resolves: #92406
Releases: main, 11.5, 10.4
Change-Id: I0b98d39f04b6c70ea370200c53316b381019dd2a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70460


Tested-by: Elias Häußler <e.haeussler@familie-redlich.de>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Elias Häußler <e.haeussler@familie-redlich.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: waldhacker <hello@waldhacker.dev>