Make shutdown polling intervals configurable

[darren-gibson]

This PR introduces configurable shutdown delays to the ActorSystem as described in #74. The following new configuration options are now available in ActorSystem.Conf:

• shutdownInitialDelay
• shutdownPollingInterval
• shutdownFinalDelay

These settings allow fine-tuning of the shutdown process, including the initial wait, polling interval for actor termination, and final delay after shutdown.

Implementation

• Added the above fields to ActorSystem.Conf with sensible defaults.
• Updated the ActorSystem.shutdown() method to use these configuration values.
• Added and refactored tests to verify:
  • The configuration values are applied correctly.
  • The shutdown operation duration reflects the configured delays.
  • Multiple test cases cover both custom and default configurations, using a shared helper to avoid code duplication.

Testing

• All new and existing tests pass.
• The shutdown duration is asserted to be within the expected range for both custom and default configurations.