Add VNet Integration for managed API in Static Web App

The need: reaching a VNet-bound resource from a managed API in Static Web App.

A VNet-bound resource could be a VM, or - more likely - a data source with a **Private Endpoint** (CosmosDB, SQL Database, Storage Account).

Today the data source must allow incoming public traffic, without even the ability of filtering by Service Tag. Most of the times this is forbidden by company policies.

If Static Web Apps could integrate into a VNet, that could solve the issue, i.e. one could block public access to the data source.