Fixes #8514 - Add support for requests to smart proxy discovery plugin

I doubt we'd get that merged into core as-is. We had a similar discussion in the template-proxying PR, and decided it didn't make sense to have code in core that depended on plugins - it breaks the whole point of extensibility.

We *could* get a patch into core that makes it easier to extend the subnet from a plugin without deface, but thats a whole bigger thing about extensible form builders :)