Gusty

Allows users to pass a custom `wait_for_class` to the `create_dag` function. The `wait_for_class` is expected to be a child class of Airflow's `ExternalTaskSensor`. Usage looks like:

python3
my_dag = create_dag(..., wait_for_class=MyCustomExternalTaskSensor)

Primarily development updates:

- `Makefile` and `Dockerfile` added for easier development

- Linted codebase (flake8)

- Bumped up minor Python versions in build matrix

- dev-requirements.txt instead of requirements.txt

- Generic parts of frontmatter parsing offloaded to [ABSQL](https://pypi.org/project/ABSQL/)

Added functionality through ABSQL integration:

- In addition to YAML constructors, users can now use Jinja-style functionality (e.g. `"{{my_func()}}"` instead of `!my_func`) in frontmatter.

- ABSQL default functions (such as `env_var`) are available to users in their task definition files.

- Similarly to `task_id` and `file_path`, gusty makes available a copy of the GustyBuilder's ABSQL runner, via `absql_runner`, for operators which accept an `absql_runner` parameter. This will give users the capability of rendering text/context with ABSQL elsewhere in their Airflow project.

Lastly, note that gusty's default parsers omit `sql` blocks from any rendering during initialization. If this becomes an issue, we can expose the ability to disable this omission in a future release.

A DAG's `wait_for_defaults` can now be overridden using `METADATA.yml`.

Introduces support for multi-task generation.

`multi_task_spec` is available in all frontmatter and can be used to generate multiple similar tasks with differing parameters between tasks.

`python_callable_partials` is generate `python_callable` partials in different tasks.

`multi_task_spec` and `python_callable_partials` can be used together.

More details are available in the README.

Now uses an internal `days_ago` function, equivalent to that in Airflow 2, as the Airflow project will eventually deprecate its own `days_ago` function.

Increases default timeout for external task sensors from 1 minute to 1 hour.