Common: add Json helper utilities for loading or saving to a file

Ensures that these go into the ro section.

We can use the string_view arguments to directly construct strings
inside of the global_result vector.

We can just use holds_alternative here instead.

VideoCommon/Statistics: Remove global system accessor from s_after_frame_event

Instead, we make the event take a reference to the system and then pass
it in when the event is triggered.

This does introduce two other accessors, but these are much easier to
refactor out over time, and without modification to the existing event
interface.

The config object is quite heavyweight, so we should move this instead
of copying.