Add support for headers in Task.apply_async()

Celery supports `headers` in `Task.apply_async()`. For example

```python
poll_tempest_crashes.apply_async(
    kwargs={"credentials_id": credentials.id},
    headers={"sentry-propagate-traces": False},
)
```

We should also support `headers` for `taskworker.Task.apply_async()`. Any provided headers are merged into the default headers generated by `Task.create_activation()`