Unable to add a VMSS to an LB backend address pool

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`az network lb address-pool create`

**Errors:**

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az network lb address-pool create`

## Expected Behavior
I expect to be able to add a vm scale set to an address pool, upon creating the address pool or through the use of the `az network lb address-pool address add` sub-command. However, it seems that this functionality is unsupported. I am only able to add individual IP addresses via `address add`.

## Environment Summary
```
macOS-10.15.6-x86_64-i386-64bit
Python 3.8.5
Installer: HOMEBREW

azure-cli 2.10.1 *

```
## Additional Context-