Connects with conf table, implements helper and adds tests

server/extension/sql/0.0.1/10_helper.sql

@@ -0,0 +1,16 @@
+-- Get values_json for provided key from conf table
+CREATE OR REPLACE FUNCTION _get_conf(_key TEXT)
+  RETURNS text
+AS $$
+DECLARE
+  rec RECORD;
+BEGIN
+  SELECT INTO rec values_json FROM conf WHERE conf.key = _key;
+
+  IF NOT FOUND THEN
+    RAISE EXCEPTION 'Missing key ''%'' in conf table', _key;
+  END IF;
+
+  RETURN rec.values_json;
+END
+$$ LANGUAGE plpgsql;