Migrate Managed Identity's implementation from msrestazure to MSAL

[jiasli]

Related command
az login --identity

Is your feature request related to a problem? Please describe.
In order to support Hybrid Runbook Worker VM's Managed Identity (#22243), Azure CLI has to use the new Managed Identity endpoint API version2019-08-01. Since msrestazure has been deprecated, we have concluded that MSAL will support Managed Identity authentication.

Describe the solution you'd like
Once MSAL finishes its implementation for Managed Identity (AzureAD/microsoft-authentication-library-for-python#480), Azure CLI needs to migrates its Managed Identity's implementation from msrestazure to MSAL.

Describe alternatives you've considered
Use Azure Identity's Managed Identity's implementation, but the dependency on Azure Identity has been removed as explained in #19853 (comment).

Additional context
Some old issues should be discussed or fixed during the migration:

• Drop Track 1 Azure SDK support #20460: Supporting Track 1 SDK adds lots of development effort. However, Storage and KeyVault command module still depends on Track 1 data-plane SDKs.
• az login with managed identity indicating system assigned when the identity is user assigned #13188

[yonzhan]

Managed Identity's implementation migration