Add code to setup permissions properly

Based on roles and by default closing everything. No need to tweak
things outside of the extension.

client/sql/90_permissions_test.sql

@@ -0,0 +1,12 @@
+-- Use regular user role
+SET ROLE test_regular_user;
+
+-- Exercise the public function
+-- it is public, it shall work
+SELECT cdb_geocoder_client.geocode_admin0_polygons('Spain');
+
+-- Check the regular user has no permissions on private functions
+SELECT cdb_geocoder_client._geocode_admin0_polygons('evil_user', 666, 'Hell');
+
+-- Check the regular user cannot look into config table
+SELECT * from cdb_geocoder_client._config;