Managed Identity Support for CosmosDB External Scaler

[ni-bhandari]

Completing the work originally started in PR #80 .

Manual testing and validation:

1. Used the Demo app in the repo to generate data in cosmos DB, used the Order Generator locally with ConnectionString.
2. Used the Demo app in the repo to process the data generated in the first step, used MSI to access the CosmosDB.
3. Created the scale object having the clientId of MSI as properties, validated the pods count increased to 1 from 0, and back to 0 after processing all the data.

The manual testing was performed on AKS cluster, details to setup the cluster with MSI is added in the Readme file of the demo app.

Here is a screengrab of the pods scaling up and down, with the order generator running in a separate window.

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)
• A PR is opened to update the documentation on our docs repo -- Seems like there aren't specific docs to update for specific external scalers.

Fixes #

1. Add support for using the KEDA external scaler with Managed Identity authentication.