Add hasRole() method to AccountProxy and UserSession classes

Problem/Motivation

UserInterface defines the hasRole() method, but the Drupal::currentUser() returns an object implementing AccountInterface, which does not have this method unless the current user is authenticated, in which case a UserInterface object is returned.

Because of this, hasRole() cannot be used without first checking the object type.

Proposed resolution

Since UserInterface extends AccountInterface, move the hasRole method there.

Quoting @catch from #28 📌 Add hasRole() method to AccountProxy and UserSession classes Fixed :

We do allow adding methods to interfaces that aren't explicitly tagged with @api, see https://www.drupal.org/about/core/policies/core-change-policies/drupal-8 → ...

However in practice, we'd want to avoid breakage in a minor for this change, so I think we should grep contrib.

If we can't change the interface in a Drupal 9 minor, we could still add the implementations of the method, and document that the additional method will be on the interface in Drupal 10 etc. just without changing the interface itself.

And @xjm:

So, I think we should do what @catch says in the second part of his comment in #28.

To summarize:
In this task, we add hasRole() implementations to AccountInterface implementations: AccountProxy and UserSession.
In the follow-up task, we add a method to AccountInterface in Drupal 10 - #3228209: Move hasRole() method from UserInterface to AccountInterface → .

Remaining tasks

Review https://git.drupalcode.org/project/drupal/-/merge_requests/1059;
Address comment #42;
Commit;

API changes

Drupal\Core\Session\AccountProxy and Drupal\Core\Session\UserSession now implement hasRole() method. See the change record: https://www.drupal.org/node/3002289 →