[Docs] Managed Identity sample for Python

[asavaritayal]

Working with Managed Identities from within a function is documented here: https://docs.microsoft.com/en-us/azure/app-service/overview-managed-identity#obtaining-tokens-for-azure-resource

There is currently no example for Python. Given that MIs are now supported on Linux App Service and Premium plans, we should provide a Python sample as well.

/cc @ggailey777

[asavaritayal]

MSI will be enabled for Linux consumption platform with ANT 83 deployment.

[anirudhgarg]

We have added a sample in Python here:
https://docs.microsoft.com/en-us/azure/app-service/overview-managed-identity#code-examples