Update snippets to initialise GraphServiceClient with different National Clouds

[Ndiritu]

Code snippets under https://learn.microsoft.com/en-us/graph/sdks/national-clouds?view=graph-rest-1.0&tabs=python need to be updated to use a TokenCredential object with an authority set and updating the base_url of the graph client via: graph_client.request_adapter.base_url = ""