feat: add a last_run field to the agent state model

[cpacker]

Please describe the purpose of this pull request.

• add a last_run field to the agent state model
  • value updated lazily (on access)
  • should use the value from messages[-1].created_at
• correct AgentStateModel's created_at to be a datetime (like the other created_at fields) instead of an int

Question for @sarahwooders - another way to do this is to keep a last_run field inside the AgentState.state that gets updated dynamically/eagerly when Agent.step() is called. Any preference on eager vs lazy?

How to test

Run an API call to list all agents (https://memgpt.readme.io/reference/list_agents_api_agents_get) and the last_run fields should exist and be valid.

Have you tested this PR?

GET localhost:8283/api/agents

{
	"num_agents": 6,
	"agents": [
		{
			"id": "1082d3e2-2e4d-40a7-9a2f-613934c3e19f",
			"name": "ResilientHarpsichord",
			"human": "basic",
			"persona": "sam_pov",
			"created_at": "2024-03-05T16:41:57.947029",
			"last_run": "2024-03-05T17:12:05.193843"
		},
...