ACI - Support 'List Cached Images' operation

**Is your feature request related to a problem? Please describe.**
It is useful to know which container images have been cached by ACI in each region.
There is an API for that so this feature request is to expose that API in a CLI command.
https://docs.microsoft.com/en-us/rest/api/container-instances/listcachedimages/listcachedimages

**Describe the solution you'd like**
A command like `az container list-cached-images -l westus` that returns:
```
[
{
"osType": "Linux",
"image": "ubuntu:16.04"
},
{
"osType": "Linux",
"image": "alpine:3.6"
},
{
"osType": "Windows",
"image": "microsoft/nanoserver:10.0.14393.2485"
}
]
```