fix: replace DefaultAzureCredential with ManagedIdentityCredential

[Priyanka-Microsoft]

Purpose

• ...
  This pull request introduces a new helper class to centralize Azure credential management and updates the configuration logic to use this helper. The changes improve maintainability and make the credential selection process more flexible based on the environment.

Azure Credential Management:

• New helper class: Added AzureCredentialHelper in App/backend-api/Microsoft.GS.DPS.Host/Helpers/AzureCredentialHelper.cs to encapsulate logic for obtaining Azure credentials. It supports both development and production environments, with an optional client ID for user-assigned managed identities.

Configuration Updates:

• Refactored App Configuration setup: Updated the Config method in AppConfiguration.cs to use AzureCredentialHelper.GetAzureCredential() instead of directly instantiating DefaultAzureCredential. This improves flexibility and aligns with the new helper class.
• Namespace import: Added a reference to Microsoft.GS.DPSHost.Helpers in AppConfiguration.cs to enable the use of the new helper class.

Does this introduce a breaking change?

• Yes
• No

Golden Path Validation

• I have tested the primary workflows (the "golden path") to ensure they function correctly without errors.

Deployment Validation

• I have validated the deployment process successfully and all services are running as expected with this change.

What to Check

Verify that the following are valid

• ...

Other Information

[github-actions]

🎉 This PR is included in version 1.0.0 🎉

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