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/src/serverless/mod.rs

@@ -11,6 +11,8 @@ mod http_conn_pool;
 mod http_util;
 mod json;
 mod local_conn_pool;
+#[cfg(feature = "rest_broker")]
+pub mod rest;
 mod sql_over_http;
 mod websocket;
 
@@ -487,6 +489,42 @@ async fn request_handler(
             .body(Empty::new().map_err(|x| match x {}).boxed())
             .map_err(|e| ApiError::InternalServerError(e.into()))
     } else {
-        json_response(StatusCode::BAD_REQUEST, "query is not supported")
+        #[cfg(feature = "rest_broker")]
+        {
+            if config.rest_config.is_rest_broker
+            // we are testing for the path to be /database_name/rest/...
+                && request
+                    .uri()
+                    .path()
+                    .split('/')
+                    .nth(2)
+                    .is_some_and(|part| part.starts_with("rest"))
+            {
+                let ctx =
+                    RequestContext::new(session_id, conn_info, crate::metrics::Protocol::Http);
+                let span = ctx.span();
+
+                let testodrome_id = request
+                    .headers()
+                    .get("X-Neon-Query-ID")
+                    .and_then(|value| value.to_str().ok())
+                    .map(|s| s.to_string());
+
+                if let Some(query_id) = testodrome_id {
+                    info!(parent: &span, "testodrome query ID: {query_id}");
+                    ctx.set_testodrome_id(query_id.into());
+                }
+
+                rest::handle(config, ctx, request, backend, http_cancellation_token)
+                    .instrument(span)
+                    .await
+            } else {
+                json_response(StatusCode::BAD_REQUEST, "query is not supported")
+            }
+        }
+        #[cfg(not(feature = "rest_broker"))]
+        {
+            json_response(StatusCode::BAD_REQUEST, "query is not supported")
+        }
     }
 }

proxy/src/serverless/sql_over_http.rs

@@ -64,7 +64,7 @@ enum Payload {
     Batch(BatchQueryData),
 }
 
-static HEADER_VALUE_TRUE: HeaderValue = HeaderValue::from_static("true");
+pub(super) const HEADER_VALUE_TRUE: HeaderValue = HeaderValue::from_static("true");
 
 fn bytes_to_pg_text<'de, D>(deserializer: D) -> Result<Vec<Option<String>>, D::Error>
 where