VS: Add a mostly-undocumented hook to load custom JSON flag tables

The names and formats of our VS flag tables are internal implementation details. However, some institutions need to maintain support for non-public VS platforms and toolsets. Provide a hook that their projects can use to load custom flag table files. This helps avoid distributing a custom CMake package within such institutions.

Document the hook itself, but explicitly specify that the files the hook loads are not considered a stable interface.