refactor: Cleanup user get_id/get_user_id (#20492)

Co-authored-by: John Bodley <john.bodley@airbnb.com>

superset/utils/core.py

@@ -1422,13 +1422,36 @@ def split_adhoc_filters_into_base_filters(  # pylint: disable=invalid-name
 
 
 def get_username() -> Optional[str]:
-    """Get username if within the flask context, otherwise return noffin'"""
+    """
+    Get username (if defined) associated with the current user.
+
+    :returns: The username
+    """
+
     try:
         return g.user.username
     except Exception:  # pylint: disable=broad-except
         return None
 
 
+def get_user_id() -> Optional[int]:
+    """
+    Get the user identifier (if defined) associated with the current user.
+
+    Though the Flask-AppBuilder `User` and Flask-Login  `AnonymousUserMixin` and
+    `UserMixin` models provide a convenience `get_id` method, for generality, the
+    identifier is encoded as a `str` whereas in Superset all identifiers are encoded as
+    an `int`.
+
+    returns: The user identifier
+    """
+
+    try:
+        return g.user.id
+    except Exception:  # pylint: disable=broad-except
+        return None
+
+
 @contextmanager
 def override_user(user: Optional[User]) -> Iterator[Any]:
     """