db: Grant read access to guest user on all tables of the schema

This now automatically grant read-only access to the guest user at db initialization or upgrade time. This way, sysadm no longer have to manually alter the schema after initialization or upgrade.

Related to swh/infra/sysadm-environment#4228 (closed)

Test Plan

tox should still be happy

---

Migrated from D7913 (view on Phabricator)

swh/core/db/tests/conftest.py

@@ -7,7 +7,9 @@ import os
 
 from click.testing import CliRunner
 from hypothesis import HealthCheck
+import psycopg2
 import pytest
+from pytest_postgresql import factories
 
 from swh.core.db.db_utils import import_swhmodule
 
@@ -21,6 +23,17 @@ function_scoped_fixture_check = (
 )
 
 
+def create_role_guest(**kwargs):
+    with psycopg2.connect(**kwargs) as conn:
+        with conn.cursor() as cur:
+            cur.execute("CREATE ROLE guest LOGIN PASSWORD 'guest'")
+
+
+postgresql_proc = factories.postgresql_proc(
+    load=[create_role_guest],
+)
+
+
 @pytest.fixture
 def cli_runner():
     return CliRunner()