Refactor SimpleTaskInstance to prevent future TaskInstance field addition to lost during serialisation

Context: #16768

> Right now, if a field is added to `TaskInstance`, `SimpleTaskInstance` and `_execute_task_callbacks` needed to be checked to pass them on. This can be difficult to maintain.

Ideas are very much welcomed.

Start with TaskInstance, SimpleTaskInstance, and _execute_task_callbacks, then read the context from issue #16768. Determine a serialization approach that does not require manually updating these paths whenever TaskInstance gains a field; done means future TaskInstance fields are not lost during serialization.

Written by the indexing model from the issue text.