Add PostgreSQL-based session management for agent memory

[syedmuneeb321]

This PR introduces a new PostgreSQLSession class that stores agent conversation history in a PostgreSQL database. This allows session-based memory to persist across restarts and supports features like message history retrieval, deletion, and clearing.

---

💡 Key Features:

• Session-Aware Memory:
  All messages are linked to a session_id, enabling multiple concurrent conversations.

• Functions Added:

  • add_items(items: list[dict]): Saves messages to the session.
  • get_items() -> list[dict]: Retrieves all messages in order.
  • pop_item() -> dict | None: Gets and deletes the last message.
  • clear_session(): Deletes all messages in the session.

---

🧪 How to Test:

1. Set the NEON_DB_URL environment variable with your PostgreSQL connection string.

2. Run the main.py script to:

   • Start a new session.
   • Add and fetch messages from PostgreSQL.

3. Optionally, call clear_session() to reset memory.

---

⚙️ Tech Stack:

• Language: Python
• Database: PostgreSQL (tested with Neon DB)
• Library: asyncpg (for fast async PostgreSQL queries)

[seratch]

Thanks for sending this. This project is going to have #1163 in future releases. So, we won't have this example in this repo.