az account set has no option to define IsDefault

**Is your feature request related to a problem? Please describe.**
Missing functionality. `az account set` does not have an option to specify the default subscription (set IsDefault), unlike PowerShell's Select-AzureSubscription command with the -Default parameter.

**Describe the solution you'd like**
support a `--default` flag for `az account set` to make it feature complete compared with PowerShell.

**Describe alternatives you've considered**
n/a

**Additional context**
This is the corresponding PowerShell command
https://docs.microsoft.com/en-us/powershell/module/servicemanagement/azure/select-azuresubscription?view=azuresmps-4.0.0