Feature: #85080 - Add property to disable form elements and finishers

Description

A a new rendering option for form elements and finishers has been introduced named enabled which takes a boolean value (true or false).

Setting enabled: true for a form element renders it in the frontend and enables processing of its value including property mapping and validation. Setting enabled: false instead disables the form element in the frontend.

Setting enabled: true for a finisher executes it when submitting forms, setting enabled: false skips the finisher instead.

By default enabled is set to true.

Usage:

All form elements and finishers except the root form element and the first form page can be enabled or disabled.

An example:

type: Form
identifier: test
label: test
prototypeName: standard
renderables:
  -
    type: Page
    identifier: page-1
    label: Step
    renderables:
      -
        type: Text
        identifier: text-1
        label: Text
        defaultValue: ''
      -
        type: Checkbox
        identifier: checkbox-1
        label: Checkbox
        renderingOptions:
          enabled: true
  -
    type: SummaryPage
    identifier: summarypage-1
    label: 'Summary step'
    renderingOptions:
      enabled: false
finishers:
  -
    identifier: Confirmation
    options:
      message: thx
  -
    identifier: Confirmation
    options:
      message: 'thx again'
      renderingOptions:
        enabled: '{checkbox-1}'

In this example the form element checkbox-1 has been enabled explicitly but it is fine to leave this out since this is the default state (which can be seen in the element text-1).

The summarypage-1 has been disabled completely, for example to temporarily remove it from the form.

The second Confirmation finisher takes the fact into account that finishers can refer to form values. It is only enabled if the form element checkbox-1 has been activated by the user. Otherwise the finisher is skipped.