[webapp]:Add support for getting list of all selectable SKUs for a given App Service Plan

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

Currently, there is no way to get a list of all the selectable SKUs for a given app service plan through Azure CLI.

**Describe the solution you'd like**

It would be handy to have a CLI command to get the list of all the selectable SKUs for a given app service plan. Like **az appservice plan list-available-sku**. The commandlet will be similar to **az vm list-sku** which lists the Virtual machine SKUs available in a region.

**Describe alternatives you've considered**

The only available option is to call Azure management REST API as explained here - https://docs.microsoft.com/en-us/rest/api/appservice/appserviceplans/getserverfarmskus

**Additional context**

I have been lately working on scaling my app services through Azure CLI command - **az appservice plan update**. It takes an input parameter called **sku**. Currently, there is no way in Azure CLI to get the list of available SKUs for the given app service plan that I can select from.