chore: Consolidate auth header creation logic

Post adding the feature of adding client-level auth headers (#178), we have the logic for creating an auth header, from the given auth token getter name, in 3 different places.

This PR unifies all of that logic into a single helper to improve maintenance, and make it easier to change the way we add suffix/prefix, and reduces WET code.

Author: anubhav756

packages/toolbox-core/tests/test_client.py

@@ -698,7 +698,6 @@ async def test_bind_param_async_callable_value_success(self, tool_name, client):
         assert res_payload == {"argA": passed_value_a, "argB": bound_value_result}
         bound_async_callable.assert_awaited_once()
 
-
     @pytest.mark.asyncio
     async def test_bind_param_success(self, tool_name, client):
         """Tests 'bind_param' with a bound parameter specified."""