Feature request: register a resource provider used in subnet delegation even if no resource in that RP is being created by the template

With [Azure Functions Flex Consumption](https://aka.ms/flexconsumption) we have an interesting situation. For the [Virtual Network Integration](https://learn.microsoft.com/en-us/azure/azure-functions/flex-consumption-how-to#enable-virtual-network-integration) feature to work, the subnet delegation needs to be against `Microsoft.App/subnet` to work. But the function app's RP is `Microsoft.Web`.

This means in a bicep like [this](https://github.com/Azure-Samples/functions-quickstart-dotnet-azd/tree/main/infra), there's no actual resource being created by the bicep against `Microsoft.Web` but [there is a subnet delegation](https://github.com/Azure-Samples/functions-quickstart-dotnet-azd/blob/f31124d2a6f9ce761966ed68a196e4df6e8ae808/infra/app/vnet.bicep#L56) against it.

RP registrations seem to already happen according to [this documentation](https://learn.microsoft.com/en-us/training/modules/child-extension-bicep-templates/2-understand-azure-resources), so this is a request for Bicep to also consider subnet delegations when auto-registering RPs.

Start with the linked infra/app/vnet.bicep example, especially the subnet delegation at line 56, and compare it with the documented existing resource-provider auto-registration behavior. Done means a template containing a subnet delegation registers the referenced provider even when it creates no other resource from that provider.

Written by the indexing model from the issue text.