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