Adding arguments to interface methods

Leveraging on Symfony's DebugClassLoader, it's now possible to add arguments to methods in interfaces in next major release.

Since it's normally a BC break to introduce new arguments (because any implementing class that does not comply with the changed signature would fail at loading), this is a two steps process.

Step 1 - prepare the new signature

During the current release cycle, introduce the new argument(s) in an inline comment, for example:

-  public function submitForm($form_arg, FormStateInterface &$form_state);
+  public function submitForm($form_arg, FormStateInterface &$form_state /* , mixed ...$args */);

Document the new argument(s) in the docblock of the method, inside an PHPCS ignore block to prevent PHPCS to fire an error when parsing the signature of the method; in the comments include a @see to a follow-up issue that will take care of actually doing the change in the next major release:

    * phpcs:disable Drupal.Commenting
    * @todo Uncomment new method parameters before drupal:11.0.0.
    * @see https://www.drupal.org/project/drupal/issues/3354672
    *
    * @param mixed ...$args
    *   Any additional arguments are passed on to the functions called by
    *   \Drupal::formBuilder()->getForm(), including the unique form constructor
    *   function. For example, the node_edit form requires that a node object is
    *   passed in here when it is called. These are available to implementations
    *   of hook_form_alter() and hook_form_FORM_ID_alter() as the array
    *   $form_state->getBuildInfo()['args'].
    * phpcs:enable

Tag the follow-up issue with 'Major version only'.

This will let the testing framework trigger deprecation errors for the implementing classes that do not have, yet, the to-be signature in place. In order to prevent tests fail because of that, add an ignore line in .deprecation-ignore.txt, like e.g.

# Drupal 11.
%FormBuilder::getForm\(\).* will require a new "mixed \.\.\. \$args" argument in the next major version of its interface%
%FormBuilder::submitForm\(\).* will require a new "mixed \.\.\. \$args" argument in the next major version of its interface%

Step 2 - implement the new signature

Once the branch opens for issues for new major release issues,

a) remove the inline comment from the interface, exposing the new full signature:

-  public function submitForm($form_arg, FormStateInterface &$form_state /* , mixed ...$args */);
+  public function submitForm($form_arg, FormStateInterface &$form_state, mixed ...$args);

b) remove the PHPCS ignore and @todo from the docblock

c) implement the new signature in the concrete classes

d) remove the ignore line from the .deprecation-ignore.txt file.