Add option to list instances of a web/function app slot

**Related command**

`az webapp deployment slot`
`az functionapp deployment slot`

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

There doesn't appear to be a method to identify web/function app slot 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 webapp deployment slot` and `az functionapp deployment slot` commands, similar to how we can already use the `az webapp list-instances` command.

I have also requested that the `list-instances` option be added to the `az functionapp` command set as well:
* #26238

**Describe alternatives you've considered**

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

**Additional context**

Web/function app slot instances can be viewed at this endpoint: `https://management.azure.com/subscriptions//resourceGroups//providers/Microsoft.Web/sites//slots//instances`