[FEATURE] Enhanced Frontend output settings

Two new options are introduced to the Frontend output FlexForm:

settings.flexform.single.disable allows the single view to be
disabled. If this checkbox is set the link to the detail view is
hidden and the detail action will redirect the user back to the
list view.

settings.flexform.main.requiredFields allows the selection of fields
that must not be empty in a mail record. Otherwise it will not be
displayed. This allows the implementation of a select field where
the user can choose if he wants his data to be displayed.

Resolves: #57952
Releases: 2.1