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/README.md

@@ -138,3 +138,62 @@ Now from client you can start a new session:
 ```sh
 PGSSLROOTCERT=./server.crt psql  "postgresql://proxy:password@endpoint.local.neon.build:4432/postgres?sslmode=verify-full"
 ```
+
+## auth broker setup:
+
+Create a postgres instance:
+```sh
+docker run \
+  --detach \
+  --name proxy-postgres \
+  --env POSTGRES_HOST_AUTH_METHOD=trust \
+  --env POSTGRES_USER=authenticated \
+  --env POSTGRES_DB=database \
+  --publish 5432:5432 \
+  postgres:17-bookworm
+```
+
+Create a configuration file called `local_proxy.json` in the root of the repo (used also by the auth broker to validate JWTs)
+```sh
+{
+    "jwks": [
+        {
+            "id": "1",
+            "role_names": ["authenticator", "authenticated", "anon"],
+            "jwks_url": "https://climbing-minnow-11.clerk.accounts.dev/.well-known/jwks.json",
+            "provider_name": "foo",
+            "jwt_audience": null
+        }
+    ]
+}
+```
+
+Start the local proxy:
+```sh
+cargo run --bin local_proxy -- \
+  --disable_pg_session_jwt true \
+  --http 0.0.0.0:7432
+```
+
+Start the auth broker:
+```sh
+LOGFMT=text OTEL_SDK_DISABLED=true cargo run --bin proxy --features testing -- \
+  -c server.crt -k server.key \
+  --is-auth-broker true \
+  --wss 0.0.0.0:8080 \
+  --http 0.0.0.0:7002 \
+  --auth-backend local
+```
+
+Create a JWT in your auth provider (e.g. Clerk) and set it in the `NEON_JWT` environment variable.
+```sh
+export NEON_JWT="..."
+```
+
+Run a query against the auth broker:
+```sh
+curl -k "https://foo.local.neon.build:8080/sql" \
+  -H "Authorization: Bearer $NEON_JWT" \
+  -H "neon-connection-string: postgresql://authenticator@foo.local.neon.build/database" \
+  -d '{"query":"select 1","params":[]}'
+```