[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/resolvers/user.ex

@@ -2,4 +2,6 @@ defmodule WabanexWeb.Resolvers.User do
   alias Wabanex.Users
 
   def get(%{id: user_id}, _context), do: Users.Get.call(user_id)
+
+  def create(%{input: params}, _context), do: Users.Create.call(params)
 end