join configuration change thread from main thread to prevent 'Runtime…

[jamhabr]

Issue #, if available:

Description of changes:

• Minor changes to Thread handling

Why is this change necessary:

• Prevent aws.greengrass.DeviceDefender: stdout. RuntimeError: can't create new thread at interpreter shutdown

How was this change tested:

• reproduced issue by waiting for the existing python script to emit 2 Device Defender metric reports with Python 3.12
• copied new changes to a local GreenGrass install
• ran ./bin/greengrass-cli component stop -n aws.greengrass.DeviceDefender and ./bin/greengrass-cli component restart -n aws.greengrass.DeviceDefender multiple times
• verified there was no error for can't create new thread at interpreter shutdown after 2 Device Metric reports were sent to Device Defender

Any additional information or context required to review the change:

Checklist:

• Updated the README if applicable
• Updated or added new unit tests
• Updated or added new integration tests
• Updated or added new end-to-end tests
• If your code makes a remote network call, it was tested with a proxy
• You confirm that the change is backwards compatible

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.