compute_ctl: catalog API endpoints (#7575)

## Problem

There are two cloud's features that require extra compute endpoints.

1. We are running pg_dump to get DB schemas. Currently, we are using a
special service for this. But it would be great to execute pg_dump in an
isolated environment. And we already have such an environment, it's our
compute! And likely enough pg_dump already exists there too! (see
https://github.com/neondatabase/cloud/issues/11644#issuecomment-2084617832)
2. We need to have a way to get databases and roles from compute after
time travel (see https://github.com/neondatabase/cloud/issues/12109)

## Summary of changes

It adds two API endpoints to compute_ctl HTTP API that target both of
the aforementioned cases.

---------

Co-authored-by: Tristan Partin <tristan@neon.tech>

libs/compute_api/src/responses.rs

@@ -3,7 +3,7 @@
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize, Serializer};
 
-use crate::spec::ComputeSpec;
+use crate::spec::{ComputeSpec, Database, Role};
 
 #[derive(Serialize, Debug, Deserialize)]
 pub struct GenericAPIError {
@@ -113,6 +113,12 @@ pub struct ComputeMetrics {
     pub total_ext_download_size: u64,
 }
 
+#[derive(Clone, Debug, Default, Serialize)]
+pub struct CatalogObjects {
+    pub roles: Vec<Role>,
+    pub databases: Vec<Database>,
+}
+
 /// Response of the `/computes/{compute_id}/spec` control-plane API.
 /// This is not actually a compute API response, so consider moving
 /// to a different place.