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>

test_runner/fixtures/endpoint/http.py

@@ -0,0 +1,23 @@
+import requests
+from requests.adapters import HTTPAdapter
+
+
+class EndpointHttpClient(requests.Session):
+    def __init__(
+        self,
+        port: int,
+    ):
+        super().__init__()
+        self.port = port
+
+        self.mount("http://", HTTPAdapter())
+
+    def dbs_and_roles(self):
+        res = self.get(f"http://localhost:{self.port}/dbs_and_roles")
+        res.raise_for_status()
+        return res.json()
+
+    def database_schema(self, database: str):
+        res = self.get(f"http://localhost:{self.port}/database_schema?database={database}")
+        res.raise_for_status()
+        return res.text