[auth_broker]: regress test (#9541)

python based regression test setup for auth_broker. This uses a http
mock for cplane as well as the JWKs url.

complications:
1. We cannot just use local_proxy binary, as that requires the
pg_session_jwt extension which we don't have available in the current
test suite
2. We cannot use just any old http mock for local_proxy, as auth_broker
requires http2 to local_proxy

as such, I used the h2 library to implement an echo server - copied from
the examples in the h2 docs.

test_runner/conftest.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 pytest_plugins = (
     "fixtures.pg_version",
     "fixtures.parametrize",
+    "fixtures.h2server",
     "fixtures.httpserver",
     "fixtures.compute_reconfigure",
     "fixtures.storage_controller_proxy",