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/compute/mod.rs

@@ -165,7 +165,7 @@ impl AuthInfo {
                 ComputeCredentialKeys::AuthKeys(AuthKeys::ScramSha256(auth_keys)) => {
                     Some(Auth::Scram(Box::new(auth_keys)))
                 }
-                ComputeCredentialKeys::JwtPayload(_) | ComputeCredentialKeys::None => None,
+                ComputeCredentialKeys::JwtPayload(_) => None,
             },
             server_params: StartupMessageParams::default(),
             skip_db_user: false,