Binding projects should be able to bind system frameworks

In https://github.com/xamarin/xamarin-macios/issues/15485 it was found that a customer did this:

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while trying to work around https://github.com/xamarin/xamarin-macios/issues/15489.

~We should warn or error when the native reference starts with "/System/Library/Frameworks/" as nothing good can come of this.~

We should make it possible to bind system frameworks.

Read the linked issues 15485 and 15489 first to understand the existing workaround and the requested binding behavior. Then trace how the shown NativeReference ItemGroup is handled in binding projects, and validate the result using an equivalent system-framework reference; done means system frameworks can be bound without the proposed warning or error.

Written by the indexing model from the issue text.