Executor Synchronous callback workload

[ferruzzi]

Add support for the Callback workload to be run in the executors. Other executors will need to be updated before they can support the workload, but I tried to make it as non-invasive as I could.

This is the bulk of the work required to allow synchronous callbacks to be used in DeadlineAlerts. For example, this now works in LocalExecutor:

with DAG(
    dag_id="sync_deadline",
    deadline=DeadlineAlert(
        reference=DeadlineReference.FIXED_DATETIME(datetime(1980, 8, 10, 2)),
        interval=timedelta(0),
        callback=SyncCallback(
            SlackWebhookNotifier,
            {"text": "Sync Callback; Alert should trigger immediately!"},
        )
    )
):
    EmptyOperator(task_id='empty_task')

Co-author: Builds on work handed off by @seanghaeli and research from @ramitkataria; if I did this right then they should be getting co-author credits, I think?

---

Was generative AI tooling used to co-author this PR?

• [ x ] Yes (please specify the tool below)
  Cline (Claude Sonnet 4.5) was used for debugging and suggesting some unit test edge cases.

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[o-nikolas]

Left some comments, but overall a large/comprehensive PR that's doing some heavy lifting. Pretty cool what's being setup here, but needs careful review by some more folks I think.

@ramitkataria should have a look through the callbacks stuff

And then maybe @jscheffl for another pair of eyes on the executor/scheduler bits?