Create a GitHub action to automatically handle dependent PRs (#7101)

This should detect when a PR has a dependency of another open PR and add
a comment and label describing it. The comment will even do a
best-effort to compute the best starting commit for review.

Whenever PRs are closed, it will also scan the open depnedent PRs and
try to either remove the PRs in the comment or if it reaches zero the
label.

It works to update a single comment on a PR rather than adding more
comments.

Assisted-by: Antigravity with Gemini

github_tools/github_helpers.py

@@ -12,7 +12,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 import argparse
 from collections.abc import Generator
 import os
-from typing import Optional
+from typing import Optional, cast
 
 # https://pypi.org/project/gql/
 import gql  # type: ignore
@@ -55,9 +55,16 @@ class Client:
         )
         self._client = gql.Client(transport=transport)
 
-    def execute(self, query: str) -> dict:
+    def execute(
+        self, query: str, variable_values: Optional[dict] = None
+    ) -> dict:
         """Runs a query."""
-        return self._client.execute(gql.gql(query))  # type: ignore
+        return cast(
+            dict,
+            self._client.execute(
+                gql.gql(query), variable_values=variable_values
+            ),
+        )
 
     def execute_and_paginate(
         self,