Generalize masquerade_target_user_access() to allow checks for any user, not just current user

Problem/Motivation

I want to create a form where I can list the users that can masquerade as a specific target user. However, masquerade_target_user_access() only has a parameter for the target user, and it assumes that I want to check the current user's access. This means I would need to copy the code into a custom function to allow checking for a different user.

Steps to reproduce

masquerade_target_user_access() doesn't have a parameter for user to check access for.

Proposed resolution

It would be simple to add a parameter and have it default to the current user if no value is passed.

Before

function masquerade_target_user_access(UserInterface $target_account) {
  $user = \Drupal::currentUser();

After

function masquerade_target_user_access(UserInterface $target_account, ?UserInterface $user = NULL) {
  if ($user === NULL) {
    $user = \Drupal::currentUser();
  }

Remaining tasks

In addition, this function should probably be moved into the service class, so that services can be injected from the container.

User interface changes

API changes

Data model changes