axum-extra: implement FromRequest for Either*

[mladedav]

Motivation

Either can currently be used only for FromRequestParts extractor. We might want to support extraction of different kinds of bodies. Example of a user wanting something like this is #3315

Solution

Have Either buffer the whole body into Bytes and then try the extractors in turn.

[jplatte]

This seems like a little bit of a footgun potentially. Using Either<Json<T>, Form<T>> is going to buffer the request body regardless of the content type.

I'm also wondering whether / how to support Either<impl FromRequest, impl FromRequestParts> or Either<impl FromRequestParts, impl FromRequest>. I'm pretty sure those don't work with this PR as-is.

Any comments / thoughts?