feat: Add created_at to Journal model

[dtatarkin]

Server returns the created_at but it is ignored by SDK, this pull request addresses this issue as created_at could be really helpful. E.g.

    {
        "id": "e364bdba-2b74-4dda-a9a0-ecbf38eedbcf",
        "entry_type": "JNLC",
        "from_account": "b91aa6b4-1805-321a-92ca-57434b652430",
        "to_account": "384ee6d9-54e1-4383-8fab-eeeb72777df8",
        "symbol": "",
        "qty": null,
        "price": "0",
        "status": "executed",
        "settle_date": "2025-03-24",
        "system_date": "2025-03-24",
        "net_amount": "10",
        "description": "Invitation reward payout",
        "currency": "USD",
        "created_at": "2025-03-22T13:41:36.468961Z"
    }```

[hiohiohio]

Thank you! Overall LGTM! Left a small comment.

[hiohiohio]

Thank you. Could you please add created_at json field [1] into the test cases [2]?

*1 something below

"created_at": "2024-05-30T09:13:03.642208Z"

*2

alpaca-py/tests/broker/broker_client/test_journal_routes.py

Line 41 in cf798a0

"currency": "USD"