Add authentication between Looker Extension and Cloud Run

[DylanBaker]

Currently, all API calls to the Cloud Run backend are unauthenticated. We should implement functionality to ensure only authorised users/services can call the endpoint.

The easiest way to do this securely is likely to save a token as a user attribute in Looker and include the token in the API calls. We can add logic to the FastAPI app to check for the token.