**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
t3.set_downstream(t5)
        t4.set_downstream(t6)
        t5.set_downstream(t6)
    Using task-decorated functions aka XComArgs:
    .. code-block:: python
        chain(x1(), [x2(), x3()], [x4(), x5()], x6())
    is equivalent to::
          / -> x2 -> x4 \
        x1               -> x6
          \ -> x3 -> x5 /
    .. code-block:: python
        x1 = x1()
        x2 = x2()
        x3 = x3()
        x4 = x4()
        x5 = x5()
        x6 = x6()
        x1.set_downstream(x2)
        x1.set_downstream(x3)
        x2.set_downstream(x4)
        x3.set_downstream(x5)
        x4.set_downstream(x6)
        x5.set_downstream(x6)
    Using TaskGroups:
    .. code-block:: python
        chain(t1, task_group1, task_group2, t2)
        t1.set_downstream(task_group1)
        task_group1.set_downstream(task_group2)
        task_group2.set_downstream(t2)
    It is also possible to mix between classic operator/sensor, EdgeModifiers, XComArg, and TaskGroups: