[Feature Request] Support Identity Federation

[jmprieur]

Is your feature request related to a problem? Please describe.

• The cert-less scenario can be generalized into an identity federation scenario.
  Implicit Azure provider in MSAL (IMDS)

Describe the solution you'd like

• Expose the notion of FederatedTokenProvider (which can be used by MSAL.NET to provide an assertion). This could be an abstract class. The issue is with how to cache the assertion. What is the key
• Rename MsiSignedAssertionProvider into AzureFederatedTokenProvider and make it public. (Maybe remove the base class)
• Expose an option EnableIdentityFederation
• Expose a property FederatedClientId, which would contain the UserAssignedManagedIdentityClientId or the system assigned managed identity clientId, and would explicitly explain that this is for the identity federation scenario.

Describe alternatives you've considered

• implicitly attempt the msi signed assertion provider when there are not credentials. But the v-team prefers things to be explicit
• Keep the UserAssignedManagedIdentityClientId
• We could also have a data structure in the configuration:

    "IdentityFederation" {
        "Enabled": true // default true
        " FederatedClientId" : Guid
     }

[jennyf19]

Released in 1.24.0.