feat: add server cache

lib/todo/cache.ex

@@ -0,0 +1,31 @@
+defmodule Todo.Cache do
+  use GenServer
+
+  @impl GenServer
+  def init(_init_args) do
+    {:ok, %{}}
+  end
+
+  @impl GenServer
+  def handle_call({:server_process, name}, _from, state) do
+    case Map.fetch(state, name) do
+      {:ok, process} ->
+        {:reply, process, state}
+
+      :error ->
+        new_process = Todo.Server.start()
+
+        new_state = Map.put(state, name, new_process)
+
+        {:reply, new_process, new_state}
+    end
+  end
+
+  def start() do
+    GenServer.start(__MODULE__, nil)
+  end
+
+  def server_process(pid, name) do
+    GenServer.call(pid, {:server_process, name})
+  end
+end