Support `az login --identity` for Azure Arc

[jimdigriz]

Is your feature request related to a problem? Please describe.
For enrolled Azure Arc systems, azure-cli does not support pulling the local managed identity.

Describe the solution you'd like
az login --identity tests if running on an Azure instance and if not to fallback to using the localhost challenge response endpoint http://localhost:40342 provided by /opt/azcmagent/bin/himds.

If azure-cli gets this functionality it means I can have my scripts call az ... directly and not have to treat Azure instances differently to on-premise kit; for example to access a shared keyvault or a workspace for logging and metrics.

Describe alternatives you've considered
I have to do the REST dance myself and spoof some credentials for the azure-cli via ~/.azure/{accessTokens,azureProfile}.json.

[yonzhan]

az login

[richeney]

@jimdigriz We definitely need this.

In the meantime, do you have any bash code that generates a valid token object in ~/.azure/accessTokens.json using the REST API?