[User Management] API improvements

[markus-moser]

• Add a fulltext search query parameter to the /api/users/tree endpoint. We need this for our search field in the frontend.
  Search for firstname, lastname, email, id like https://github.com/pimcore/admin-ui-classic-bundle/blob/1.x/src/Controller/Admin/UserController.php#L932
  Will be a new user/search endpoint.

• Add the user references/dependencies to the detail endpoint. We need this for our "This user is referenced in the following objects" tab:
  
  It should be the same data like in the objectDependencies attribute of our user detail endpoint from the classic bundle (hasHidden flag + dependencies):
  

• Updating the keyBindings currently produces an error. We send the data the same way like we receive it from the get endpoint:
  

• For a user without permissions (directly after creating a new one) we currently get a permission array with one single empty string as entry. Please change it to an empty array: