feat: Add created_at to workflow_run_jobs schema

[santhosh-fit]

Summary

This pull request adds the created_at property to the workflow_run_jobs schema. This allows users to extract the creation time of workflow run jobs and would be helpful in the tracking individual jobs queue_time.

Changes

• Updated repository_streams.py to include created_at in the schema definition of the workflow_run_jobs stream.

Testing

1. Confirmed that Github REST API endpoint for workflow jobs response returns created_at field in the response.
2. Tested the changes locally using Meltano to ensure the created_at field is correctly included and populated.

[edgarrmondragon]

Thanks you @santhosh-fit!