**Description**

It would be nice if the weight_rule default value could be managed by a global config

suggested config:
```
# Weighting method used for the effective total priority weight of the task. 
# Options are: { downstream | upstream | absolute } default is
default_weight_rule = downstream
```
**Use case / motivation**

In some pipeline, you really need to have absolute weight, and then you have to add a line in each task definition which is annoying

airflow/models/baseoperator.py
.. code-block:: python
        chain(t1, [Label("branch one"), Label("branch two")], [x1(), x2()], task_group1, t2())
    is equivalent to::
          / "branch one" -> x1 \
        t1                      -> t2 -> x3
          \ "branch two" -> x2 /
    .. code-block:: python
        x1 = x1()
        x2 = x2()
        x3 = x3()
        label1 = Label("branch one")
        label2 = Label("branch two")
        t1.set_downstream(label1)
        label1.set_downstream(x1)
        t2.set_downstream(label2)
        label2.set_downstream(x2)
        x1.set_downstream(task_group1)
        x2.set_downstream(task_group1)
        task_group1.set_downstream(x3)
        # or
        x1 = x1()
        x2 = x2()
        x3 = x3()
        t1.set_downstream(x1, edge_modifier=Label("branch one"))
        t1.set_downstream(x2, edge_modifier=Label("branch two"))
        x1.set_downstream(task_group1)
        x2.set_downstream(task_group1)
        task_group1.set_downstream(x3)
    :param tasks: Individual and/or list of tasks, EdgeModifiers, XComArgs, or TaskGroups to set dependencies
    :type tasks: List[airflow.models.BaseOperator], airflow.models.BaseOperator,