Feature: #83405 - add ConfirmationFinisher template

Description

The ConfirmationFinisher message is now rendered within a fluid template to allow styling of the message. Furthermore, the FormRuntime (and thus all form element values) and the finisherVariableProvider are available in the template [1]. Custom variables can be added globally within the form setup or at form level in the form definition [2]. By using a fluid template and the associated html escaping, the display of the ConfirmationFinisher message is protected against XSS / html injection attacks. The ext: form supplied fluid template does not include any HTML wrapping to remain compatible with existing installations, but it is possible to implement your own template [3].

[1] Template variables

{form} - Object for access to submitted form element values (https://docs.typo3.org/typo3cms/extensions/form/Concepts/FrontendRendering/Index.html#accessing-form-values)
{finisherVariableProvider} - Object with data from previous finishers (https://docs.typo3.org/typo3cms/extensions/form/Concepts/FrontendRendering/Index.html#share-data-between-finishers)
{message} - The confirmation message

[2] custom template variables

global within the form setup:

TYPO3:
  CMS:
    Form:
      prototypes:
        standard:
          finishersDefinition:
            Confirmation:
              options:
                variables:
                  foo: bar

per form within the form definition:

finishers:
  -
    identifier: Confirmation
    options:
      message: 'Thx'
      variables:
        foo: bar

[3] custom Template

form setup:

TYPO3:
  CMS:
    Form:
      prototypes:
        standard:
          finishersDefinition:
            Confirmation:
              options:
                templateRootPaths:
                  20: 'EXT:my_site_package/Resources/Private/Templates/Form/Finishers/Confirmation/'

Impact

Integrators can use a ConfirmationFinisher message within a fluid template. Integrators can use additional information such as form element values within the template. The ConfirmationFinisher message is protected against XSS / html injection attacks.