Core: Remove class_db.h include from ref_counted.h

[Repiteo]

• Helps address [TRACKER] Compile time issues caused by excessive includes #111218

Changes:

• Ref templates for PtrToArg, GetTypeInfo, and VariantInternalAccessor were migrated to the original headers, as there's no longer a risk of circular includes.
• Headers with functions operating on ClassDB directly had their logic migrated to implementation files.

I know there's been hesitation in the past about tackling the removal of class_db.h includes¹, but I think the changes here demonstrate that this warrants revisiting. Even after migrating ClassDB logic to implementation files, many headers & implementation files alike still needed additional includes. This is because several of the affected files weren't for Object subclasses at all; plenty of utilities were including class_db.h indirectly!

This isn't even mentioning the current cost of these includes. class_db.h and ref_counted.h both rank in the top 5 of fresh compile and recompile times. While Object and Variant are likely the two "real" compile-time culprits, Ref wasn't helping matters with a direct include of ClassDB. So while it'll likely still rank fairly high, it's feasible that this change alone could dethrone ref_counted.h as the single most expensive file from a fresh compilation.

Footnotes

1. https://github.com/godotengine/godot/pull/111573#pullrequestreview-3330940683 ↩

[Ivorforce]

LGTM