Add helper methods to run easily run extractors

[davidpdrsn]

In axum 0.5 you could call request_parts.extract::<T>() which is a bit nicer than T::from_request(request_parts) but #1272 removed the RequestParts type and thus also this method.

For 0.6 we should think about adding back something similar.

Haven't thought much about it but perhaps something like

pub trait RequestExt {
    async fn extract<T>(self) -> Result<(Self, T), T::Rejection>
    where
        T: FromRequest,
    {
        ...
    }
}

pub trait PartsExt {
    async fn extract<T>(&mut self) -> Result<T, T::Rejection>
    where
        T: FromRequestParts,
    {
        ...
    }
}

Should also think about how to pass state and whether we need separate methods for when the state is and isn't ().

[jplatte]

We should also have extract_parts in RequestExt, so users don't need to do into_parts + from_parts themselves. Although calling that multiple times consecutively might be slower than doing one into_parts + from_parts and using .extract on the Parts.