[FEATURE REQ] Azure web app swap preserve vnet

**Is your feature request related to a problem? Please describe.**

When I use RM PowerShell to swap slots for Azure Web app, there is parameter called PreserveVnet ,[details](https://docs.microsoft.com/en-us/powershell/module/azurerm.websites/switch-azurermwebappslot?view=azurermps-6.13.0). And it is also available in the [REST API](https://docs.microsoft.com/en-us/rest/api/appservice/webapps/swapslotslot#request-body). But the sdk here does not provide such parameter which makes me confused.

**Describe the solution you'd like**
Enable the parameter in the sdk or describe the why there is no that parameter.

**Describe alternatives you've considered**
None.

**Additional context**
None.

**Information Checklist**
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- [X] Expected solution specified