enhancement: (#2934) Allows flatten attribute to work for Optional values when deriving FromRow

[jonnyso]

fixes #2934

[abonander]

This change appears innocuous enough, but I'm worried that people won't understand what this impl is for and will assume it does something different, i.e. make .fetch_one() return None instead of erroring on an empty resultset. That's not really possible with our current API design, which makes this impl a potential footgun since it'd compile but behave differently than expected at runtime.

I think I'd prefer if this was something much more specific to #[sqlx(flatten)], and I think we can accomplish that without adding a new public blanket impl, using a trick called autoderef specialization: http://lukaskalbertodt.github.io/2019/12/05/generalized-autoref-based-specialization.html

I used the same trick in #3356 although the solution I'm imagining isn't going to look exactly like that.

[abonander]

(As for why to use autoderef specialization instead of just checking if the type is wrapped in Option: that can't see through type aliases.)

[jonnyso]

I agree it would be inconsistent with the way .fetch_one() works right now, I'll look into the suggestions you made, thanks !