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/auth/backend/mod.rs

@@ -171,7 +171,6 @@ impl ComputeUserInfo {
 pub(crate) enum ComputeCredentialKeys {
     AuthKeys(AuthKeys),
     JwtPayload(Vec<u8>),
-    None,
 }
 
 impl TryFrom<ComputeUserInfoMaybeEndpoint> for ComputeUserInfo {