For documentation, see:
1. Docstring of `task.task`
2. Tests in `tests.test_task` e.g. `test_retry_as_per_task_definition`
3. Sample usages in `tests.apps.simple_app` e.g. `append_to_file`

Changelist:

* Formalize serialization and deserialization
* Serialize & deserialize exceptions correctly
* Encapsulate retry & retry_on in a new dict 'options'
* Implement serde for AsyncResult
* Ensure generated file deleted after test
* Add jsonpickle to toml file
* Exclude `if TYPE_CHECKING:` from coverage
* Add test for singleton
* Add logging for worker
* Wrap all constants inside `Config` class

Signed-off-by: Imran Ariffin <[email protected]>