Interceptors still require InterceptorsPreviewNamespaces property

[captainsafia]

The compiler will emit a warning for source generators that use interceptors about the need for an InterceptorsPreviewNamespaces property in the target project.

/Users/captainsafia/git/aspnetcore/openapi-fixes/artifacts/obj/Microsoft.AspNetCore.OpenApi/Debug/net9.0/Microsoft.AspNetCore.Http.RequestDelegateGenerator/Microsoft.AspNetCore.Http.RequestDelegateGenerator.RequestDelegateGenerator/GeneratedRouteBuilderExtensions.g.cs(61,10): error CS9137: The 'interceptors' experimental feature is not enabled in this namespace. Add '<InterceptorsPreviewNamespaces>$(InterceptorsPreviewNamespaces);Microsoft.AspNetCore.Http.Generated</InterceptorsPreviewNamespaces>' to your project.

cc: @jaredpar @RikkiGibson

[RikkiGibson]

My expectation here is that we would introduce an InterceptorsNamespaces property which is basically just an alias for InterceptorsPreviewNamespaces, at least in the medium term.

The reason is that this simplifies "discovery" of [InterceptsLocation] attributes which has to occur when public APIs like SemanticModel.GetInterceptorMethod(InvocationExpressionSyntax) are used.

Essentially until we make the engineering investment to make discovery more performant for editor scenarios it feels like we should continue to require this.