VMSS: Add support for retrieveBootDiagnosticsData boot log

**Related command**
```
az vm boot-diagnostics get-boot-log
```

**Is your feature request related to a problem? Please describe.**
I'm am investigating kernel panics with VM's that are part of a uniform VMSS. To do so, I need to review the serial console logs. While this is possible in the Azure portal, it's extremely tedious to click through into the right section per host.

**Describe the solution you'd like**
I would like to see the addition of a command like:

```
az vmss boot-diagnostics get-boot-log
```
That would query the `retrieveBootDiagnosticsData` API endpoint and fetch the boot log just like the existing command does for the `az vm` command.

**Describe alternatives you've considered**
I am using the `az rest` call to directly hit the API instead.

**Additional context**
Azure Support Case #2504170040012410