[proxy] Forward compute connection params to client

This fixes all kinds of problems related to missing params,
like broken timestamps (due to `integer_datetimes`).

This solution is not ideal, but it will help. Meanwhile,
I'm going to dedicate some time to improving connection machinery.

Note that this **does not** fix problems with passing certain parameters
in a reverse direction, i.e. **from client to compute**. This is a
separate matter and will be dealt with in an upcoming PR.

test_runner/regress/test_proxy.py

@@ -122,3 +122,33 @@ def test_auth_errors(static_proxy: NeonProxy):
     # Finally, check that the user can connect
     with static_proxy.connect(user="pinocchio", password="magic", options="project=irrelevant"):
         pass
+
+
+def test_forward_params_to_client(static_proxy: NeonProxy):
+    # A subset of parameters (GUCs) which postgres
+    # sends to the client during connection setup.
+    # Unfortunately, `GUC_REPORT` can't be queried.
+    # Proxy *should* forward them, otherwise client library
+    # might misbehave (e.g. parse timestamps incorrectly).
+    reported_params_subset = [
+        "client_encoding",
+        "integer_datetimes",
+        "is_superuser",
+        "server_encoding",
+        "server_version",
+        "session_authorization",
+        "standard_conforming_strings",
+    ]
+
+    query = """
+        select name, setting
+        from pg_catalog.pg_settings
+        where name = any(%s)
+    """
+
+    with static_proxy.connect(options="project=irrelevant") as conn:
+        with conn.cursor() as cur:
+            cur.execute(query, (reported_params_subset,))
+            for name, value in cur.fetchall():
+                # Check that proxy has forwarded this parameter.
+                assert conn.get_parameter_status(name) == value