[day-03] add mutation to create user

1. Add input type to receive user details
2. Add resolver to create user
3. Configure mutation in root type
4. Aggregate mutations in schema

lib/wabanex_web/schema.ex

@@ -6,4 +6,8 @@ defmodule WabanexWeb.Schema do
   query do
     import_fields :root_query
   end
+
+  mutation do
+    import_fields :root_mutation
+  end
 end