Gusty

external_dependencies` blocks now accept dicts, where the keys are the external DAG names and the values are either:

- `tasks`: A list of tasks out of which ExternalTaskSensors should be created
- ExternalTaskSensor parameters, such as `retries` or `execution_delta`, so that users can override any `wait_for_defaults` set on the DAG-level.

`external_dependencies` dicts are available at the task, task group, and DAG level.

Additionally, a bug was fixed, so now multiple tasks from one external DAG can be listed in `external_dependencies` blocks. Note that list-style `external_dependencies` are permissible.

Custom `dag_constructors` now supported on all file types.

Supports passing a list of functions to the `dag_constructors` argument of `create_dag`.

Adds support for custom YAML constructors in `create_dag` by passing a dict of constructor tags and functions (e.g. `{"!custom_func": custom_func}`) to the `dag_constructors` argument of `create_dag`.

Replaces `GustyYAMLLoader` class with `generate_loader` function.

Updated the `GustyYAMLLoader`

- Uses SafeLoader
- Updated and tested to work with all constructor functions (`!days_ago`, `!timedelta`, and `!datetime`)

Added `NonexistentDagDirError` to give a clear error message when creating a DAG from a directory that does not exist.