Support running extractors from middleware::from_fn

[davidpdrsn]

Lately we've had a few people ask about how run extractors from middleware. This was previously possible by building your own RequestParts and then running the extractor. But this adds support to middleware::from_fn to just support extractors directly. I think that is quite clean and doesn't require any new API.

It means if people are writing a middleware by implementing Service themselves then they still have to do it with RequestParts. I don't think thats too bad but we can always consider improving that in the future. I still think this change makes sense on its own though.

Its a breaking change because some type params and bounds changed.

Supersedes #1083

[jplatte]

I guess if we do this for the error-handling layer too, it's fine (and can always be improved upon later).

[davidpdrsn]

I have no idea what happened to this but the commits weren't in main 🤷 So I've cherry-picked them and verified that the other things merged into axum-next are there.