Feature: #82706 - Render fieldset labels in form templates

See forge#82706


The section element Fieldset is now accessible in templates of the "form" extension and can be used to add more structure.

By default this affects the SummaryPage form element as well as the EmailToReceiver / EmailToSender finishers.

A common use case are two fieldsets for a delivery and a billing address where the fields within the fieldset are usually named the same. Till now, the default mail sent by the "form" extension would e.g. show "Street" twice without giving a hint about the context. Now the fieldset label is rendered in between to separate those fields.


The summary page of the "form" extension and mails now show fieldset labels as separators.

Custom templates have access to the fieldset element for rendering.