feat: Add created_at to workflow_run_jobs schema (#275)

### 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.

tap_github/repository_streams.py

@@ -2047,6 +2047,7 @@ class WorkflowRunJobsStream(GitHubRestStream):
         th.Property("html_url", th.StringType),
         th.Property("status", th.StringType),
         th.Property("conclusion", th.StringType),
+        th.Property("created_at", th.DateTimeType),
         th.Property("started_at", th.DateTimeType),
         th.Property("completed_at", th.DateTimeType),
         th.Property("name", th.StringType),