subzero pre-integration refactor (#12416)

## Problem
integrating subzero requires a bit of refactoring. To make the
integration PR a bit more manageable, the refactoring is done in this
separate PR.
 
## Summary of changes
* move common types/functions used in sql_over_http to errors.rs and
http_util.rs
* add the "Local" auth backend to proxy (similar to local_proxy), useful
in local testing
* change the Connect and Send type for the http client to allow for
custom body when making post requests to local_proxy from the proxy

---------

Co-authored-by: Ruslan Talpa <ruslan.talpa@databricks.com>

proxy/src/serverless/mod.rs

@@ -29,13 +29,13 @@ use futures::future::{Either, select};
 use http::{Method, Response, StatusCode};
 use http_body_util::combinators::BoxBody;
 use http_body_util::{BodyExt, Empty};
+use http_util::{NEON_REQUEST_ID, uuid_to_header_value};
 use http_utils::error::ApiError;
 use hyper::body::Incoming;
 use hyper_util::rt::TokioExecutor;
 use hyper_util::server::conn::auto::Builder;
 use rand::SeedableRng;
 use rand::rngs::StdRng;
-use sql_over_http::{NEON_REQUEST_ID, uuid_to_header_value};
 use tokio::io::{AsyncRead, AsyncWrite};
 use tokio::net::{TcpListener, TcpStream};
 use tokio::time::timeout;