Add special handling of Extension and Clone for debug_handler

[loganbnielsen]

Motivation

Addresses #2294 using the solution discussed there. (Link)

[loganbnielsen]

Am I supposed to do anything to request a review?

[jplatte]

No. I can request David's review and maybe also take a look myself (though given that this is mostly stuff that David wrote, I don't want to merge w/o his review), but I don't think it will actually speed up things as there's probably a bandwidth problem, not an awareness problem.

[loganbnielsen]

No problem, just wanted to make sure I wasn't dropping the ball

[yanns]

This can have a good value when someone is using Extension.

[jplatte]

I couldn't see why we would need to special case Extension in particular when we already make sure to have good error messages for other extractors such as Json so I decided to see what the error message is on main with #[debug_handler]. Turns out that we don't actually need to do anything, see #2731.