Azure.Identity: Add ChainedTokenCredential Support

[scottaddie]

Add support for ChainedTokenCredential in ConfigurableCredentialProvider.

Enable this credential to be resolved via CredentialSource in configuration.

Example configuration:

{
  "Credential": {
    "CredentialSource": [
      "VisualStudio",
      "AzureCli"
    ]
  }
}