[Feature Request] Adding a "Show Crosshair" setting so the frontend itself could display a crosshair for light gun games

Description

Hey!

So a big problem with light gun games is that not all cores draw a crosshair, the best example is probably with PS1, we have 4 cores (Beetle PSX/HW, PCSX ReARMed and DuckStation) and only Beetle PSX (or HW in software mode...) is drawing a crosshair, and not a very sexy one... 😅

So the idea would be to have a "Show Crosshair" setting or something in RetroArch directly to display a crosshair, this way if a core doesn't draw a crosshair you'll have an alternative!

Icing on the cake would be for the feature to accept custom .png made by the user.

Expected behavior

If a core doesn't draw a crosshair, just turn ON "Show Crosshair" (in On-Screen Display menu?) and tadaaa!

Actual behavior

It makes the game pretty much unplayable since you have no idea where your cursor is between 2 shots...

Steps to reproduce the bug

1. Start Time Crisis with Beetle PSX HW (not in software mode), PCSX ReARMed or DuckStation.
2. Set device type to Guncon.
3. Try to play while mouse is captured, good luck!

Tagging @Sanaki as requested :)

The issue does not name implementation files, tests, or entry points. Begin by reviewing the proposed frontend setting in the On-Screen Display menu and the listed light-gun reproduction cases; done means a core-independent crosshair can be enabled and displayed, with custom PNG support clarified before implementation.