feat(hc): Add __str__ implementation for RpcUser (#58485)

This matches the implementation in
[`django.contrib.auth.base_user.AbstractBaseUser`](https://github.com/django/django/blob/main/django/contrib/auth/base_user.py#L54-L55).

Project transfer emails specifically rely upon this behavior, but there
are likely other usages:
-
https://github.com/getsentry/sentry/blob/master/src/sentry/api/endpoints/project_transfer.py#L88
-
https://github.com/getsentry/sentry/blob/master/src/sentry/templates/sentry/emails/transfer_project.txt#L5

Fixes HC-919

Author: mrduncan

src/sentry/services/hybrid_cloud/user/model.py

@@ -68,6 +68,9 @@ def __hash__(self) -> int:
         # TODO: Remove the need for this
         return hash((self.id, self.pk))
 
+    def __str__(self):  # API compatibility with ORM User
+        return self.get_username()
+
     def by_email(self, email: str) -> "RpcUser":
         if email == self.email:
             return self