VMSS: Add support for retrieveBootDiagnosticsData boot log

[jaredledvina]

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

[yonzhan]

Thank you for opening this issue, we will look into it.