Fix missing pool_config import issue and provide default database pool configuration.

[KarthikeyaKollu]

Description

This PR resolves the issue of a missing pool_config module in the SQLAlchemyDatabaseManager implementation (memori/database/sqlalchemy_manager.py). The absence of this module caused an ImportError.

To address the issue, I added a new pool_config.py file to centralize the default database connection pool settings. The following configurations have been introduced:

• DEFAULT_POOL_SIZE = 5: Sets the maximum size of the connection pool for regular database connections.
• DEFAULT_MAX_OVERFLOW = 10: Defines the maximum number of connections that can exceed the pool size during high-demand periods.
• DEFAULT_POOL_TIMEOUT = 30: Specifies the time (in seconds) the application waits for a database connection before throwing a timeout error.
• DEFAULT_POOL_RECYCLE = 3600: Ensures that connections are refreshed/recycled after 1 hour to prevent stale or idle connections.
• DEFAULT_POOL_PRE_PING = True: Enables pre-ping checks to validate connection health before use.