build(repo): structure the repo into postgresql subdirectory with separate template and internal tests

postgresql/tests/init_extra_users.sql

@@ -0,0 +1,5 @@
+-- Create additional user for partitioning tasks
+CREATE USER zbx_part WITH PASSWORD 'zbx_part';
+GRANT CONNECT ON DATABASE zabbix TO zbx_part;
+-- Grant usage on public schema (standard for PG 15+)
+GRANT USAGE ON SCHEMA public TO zbx_part;