Add option to list instances of a function app

[jaredbrogan]

Related command

az functionapp

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

There doesn't appear to be a method to identify function app instances outside of navigating to https://resources.azure.com

Describe the solution you'd like

I'd like to see a new list-instances option added to the az functionapp command, similar to how we can already use the az webapp list-instances command.

Describe alternatives you've considered

Using https://resources.azure.com, which is not preferred or desirable.

Additional context

Function app instances can be viewed at this endpoint: https://management.azure.com/subscriptions/<SUB_ID>/resourceGroups/<RSG_NAME>/providers/Microsoft.Web/sites/<APP_NAME>/instances

[yonzhan]

Thank you for opening this issue, we will look into it.