Add code to setup permissions properly

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

client/expected/00_installation_test.out

@@ -12,3 +12,7 @@ SELECT cdb_geocoder_client._config_set('db_server_config', '{"connection_str": "
 
 -- Mock the server schema
 CREATE SCHEMA cdb_geocoder_server;
+-- Create a test user to check permissions
+DROP ROLE IF EXISTS test_regular_user;
+CREATE ROLE test_regular_user;
+GRANT publicuser TO test_regular_user;

client/expected/90_permissions_test.out

@@ -0,0 +1,19 @@
+-- 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');
+NOTICE:  cdb_geocoder_client._geocode_admin0_polygons(3): [contrib_regression] REMOTE NOTICE: cbd_geocoder_server.geocode_admin0_polygons invoked with params (postgres, some_transaction_id, Spain)
+CONTEXT:  SQL statement "SELECT cdb_geocoder_client._geocode_admin0_polygons(session_user, txid_current(), country_name)"
+PL/pgSQL function cdb_geocoder_client.geocode_admin0_polygons(text) line 5 at SQL statement
+ geocode_admin0_polygons 
+-------------------------
+ 
+(1 row)
+
+-- Check the regular user has no permissions on private functions
+SELECT cdb_geocoder_client._geocode_admin0_polygons('evil_user', 666, 'Hell');
+ERROR:  permission denied for function _geocode_admin0_polygons
+-- Check the regular user cannot look into config table
+SELECT * from cdb_geocoder_client._config;
+ERROR:  permission denied for relation _config