[Feature Request] MSAL supports Managed Identity

[neha-bhargava]

Add MSI code to MSAL from Azure.Identity repo.

• Initial work with app service nanaged identity source
• Add code for imds source
• Add code for azure arc source
• Add code for service fabric source - 3 days
• Add code for cloud shell source
• Disable instance discovery for managed identity / AppTokenProvider using the flag
• Add integration tests - 1 week
• Document manual testing for cloud shell / service fabric - 1 week
• Handle error scenarios when no MSI endpoint is available.
• Handle error codes from imds endpoint 404, 410: when it is upgrading.
• Add retry logic for managed identity flow #4067
• Compute perf impact of caching or not caching the ManagedIdentitySource object
• API review documentaion - 2 days
• Add telemetry using the telemetry client for managed identity - 2 days
• Redesign the Managed Identity public API #3970
• E2E testing - 1 week
• [ ] UAT - 2 weeks
• Documentation - 1 week
• Performance analysis between MSAL and Azure SDK
• Distributed cache solution for managed identity #4042
• Create ManagedIdentityException to send back the source in the exception #4041
• [Feature Request] Managed Identity - Calculate the refresh_in value based on expires_in #4062 (3 days)
• [Feature Request] Managed Identity - Add a web app sample to demonstrate MI in MSAL  #4063 (1 week)
• Custom exception when IMDS endpoint can't be reached
• Update managed identity API to specify the identity when creating the application. #4114
• GA Managed Identity feature #4125

As discussed with the MSI team:

• We don't need to implement the AppServiceV2019ManagedIdentitySource and TokenExchangeManagedIdentitySource
• We need to support Resource_id as a parameter to the API in addition to UserAssignedClientId. User can provide any of them but not both.

[bgavrilMS]

@neha-bhargava - should we use this work item to track the entire feature (except testing?). Can you please expand on work remaining (e.g. spec, missing API, exceptions etc.)