Helpers to transform Twisted requests to Rust http Requests/Responses

[sandhose]

Needed for #17056

This adds functions to transform a Twisted request to the http::Request, and then to send back an http::Response through it.

It also imports the SynapseError exception so that we can throw that from Rust code directly

Example usage of this would be:

use crate::http::{http_request_from_twisted, http_response_to_twisted, HeaderMapPyExt};

fn handler(twisted_request: &PyAny) -> PyResult<()> {
    let request = http_request_from_twisted(twisted_request)?;

    let ua: headers::UserAgent = request.headers().typed_get_required()?;

    if whatever {
        return Err((crate::errors::SynapseError::new(
            StatusCode::UNAUTHORIZED,
            "Whatever".to_owned
            "M_UNAUTHORIZED",
            None,
            None,
        )));
    }

    let response = Response::new("hello".as_bytes());
    http_response_to_twisted(twisted_request, response)?;

    Ok(())
}

[anoadragon453]

lgtm overall! Just some small changes really.

[anoadragon453]

Thanks for the changes. This looks good to me!