webapp:Set-AzWebApp / Set-AzWebAppSlot support Managed User Identity

## Description of the new feature
Currently the setting of the Identity for an App/Slot only allows a System Assigned identity
https://github.com/Azure/azure-powershell/blob/51bb40ad4aae1a150556829f67bf405dea50534c/src/Websites/Websites/Cmdlets/WebApps/SetAzureWebApp.cs#L276

It would be ideal if there was a way to assign a User Managed Identity

## Proposed implementation details (optional)
Maybe in the same parameter set at

https://github.com/Azure/azure-powershell/blob/51bb40ad4aae1a150556829f67bf405dea50534c/src/Websites/Websites/Cmdlets/WebApps/SetAzureWebApp.cs#L123-L124

```
[Parameter(ParameterSetName = ParameterSet1Name, Mandatory = false, HelpMessage = ", separated list of Managed User Identities")]
public string[] UserManagedIdentities { get; set; }
```

if UserManagedIdentities is empty or null, then assign a system managed identity.
(or provide an additional property IdentityType (system,user)