Make array of iframe buster configs accessible in a generic way

It would be useful for other code to have access to the array of supported iframe busters, for example, to see if a file is supported by the plugin or not.

Should provide a method `wpcom_vip_get_supported_ad_busters()` (or refactor into a class).

See https://github.com/Automattic/vip-scanner/issues/31

Start by locating the array of supported iframe buster configurations and the plugin entry point that currently uses it. Check the related vip-scanner issue for context, then expose the supported configurations through wpcom_vip_get_supported_ad_busters() or the agreed class interface; done means other code can inspect whether a file is supported.

Written by the indexing model from the issue text.