proxy: subzero integration in auth-broker (embedded data-api) (#12474)

## Problem
We want to have the data-api served by the proxy directly instead of
relying on a 3rd party to run a deployment for each project/endpoint.

## Summary of changes
With the changes below, the proxy (auth-broker) becomes also a
"rest-broker", that can be thought of as a "Multi-tenant" data-api which
provides an automated REST api for all the databases in the region.

The core of the implementation (that leverages the subzero library) is
in proxy/src/serverless/rest.rs and this is the only place that has "new
logic".

---------

Co-authored-by: Ruslan Talpa <ruslan.talpa@databricks.com>
Co-authored-by: Alexander Bayandin <alexander@neon.tech>
Co-authored-by: Conrad Ludgate <conrad@neon.tech>

proxy/subzero_core/.gitignore

@@ -0,0 +1,2 @@
+target
+Cargo.lock

proxy/subzero_core/Cargo.toml

@@ -0,0 +1,12 @@
+# This is a stub for the subzero-core crate.
+[package]
+name = "subzero-core"
+version = "3.0.1"
+edition = "2024"
+publish = false # "private"!
+
+[features]
+default = []
+postgresql = []
+
+[dependencies]

proxy/subzero_core/src/lib.rs

@@ -0,0 +1 @@
+// This is a stub for the subzero-core crate.