[FEATURE] Introduce ignoreFlexFormSettingsIfEmpty extbase configuration

Extbase extension settings defined in FlexForm always override settings
defined globally in TypoScript. This is a problem for FlexForm fields
which are empty, since an empty value will always override a possible
globally defined TypoScript setting. This behavior might be unwanted
and in case of EXT:felogin, it is currently not possible to define
various settings in TypoScript (e.g. redirect PIDs, storage PID and
more), since they are unintentionally overridden by FlexForm settings.

This patch introduces the new extbase TypoScript configuration
`ignoreFlexFormSettingsIfEmpty`, which allows extbase extensions
to define FlexForm settings, that will be ignored if empty.

Additionally, the new PSR-14 event
`BeforeFlexFormConfigurationOverrideEvent` is available for extension
authors, so a custom FlexForm override process can be implemented.

The new feature is now used in ext:felogin to prevent unwanted
overrides of globally defined TypoScript extension settings.

Resolves: #99976
Related: #92363
Related: #61649
Releases: main
Signed-off-by: Torben Hansen <derhansen@gmail.com>
Change-Id: Ie565276d4cb0850f041ca2faead76f44e5e578c0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77885


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Alexander Schnitzler <git@alexanderschnitzler.de>