Support Installed code via python callable string (#221)

i want to be able to define a path to a custom function that i am
installing into my airflow environment. This will allow me to test my
dag locally without having to define a path and simply refer to the
package path instead.

Example

```yaml
        branch_by_bq_load_status:
            operator: airflow.operators.python.BranchPythonOperator
            python_callable: custom.path.to.func

```

---------

dagfactory/dagbuilder.py

@@ -321,6 +321,8 @@ def make_task(operator: str, task_params: Dict[str, Any]) -> BaseOperator:
                     # Airflow 2.0 doesn't allow these to be passed to operator
                     del task_params["python_callable_name"]
                     del task_params["python_callable_file"]
+                elif isinstance(task_params["python_callable"], str):
+                    task_params["python_callable"]: Callable = import_string(task_params["python_callable"])
 
             # Check for the custom success and failure callables in SqlSensor. These are considered
             # optional, so no failures in case they aren't found. Note: there's no reason to

tests/test_dagbuilder.py

@@ -354,7 +354,18 @@ def test_make_python_operator():
     assert callable(actual.python_callable)
     assert isinstance(actual, PythonOperator)
 
-
+def test_make_python_operator_with_callable_str():
+    td = dagbuilder.DagBuilder("test_dag", DAG_CONFIG, DEFAULT_CONFIG)
+    operator = "airflow.operators.python_operator.PythonOperator"
+    task_params = {
+        "task_id": "test_task",
+        "python_callable": "builtins.print",
+    }
+    actual = td.make_task(operator, task_params)
+    assert actual.task_id == "test_task"
+    assert callable(actual.python_callable)
+    assert isinstance(actual, PythonOperator)
+
 def test_make_python_operator_missing_param():
     td = dagbuilder.DagBuilder("test_dag", DAG_CONFIG, DEFAULT_CONFIG)
     operator = "airflow.operators.python_operator.PythonOperator"