**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
        cross_downstream(from_tasks=[t1, x2(), t3], to_tasks=[x1(), t2, x3()])
    is equivalent to::
        t1 ---> x1
           \ /
        x2 -X -> t2
           / \
        t3 ---> x3
    .. code-block:: python
        x1 = x1()
        x2 = x2()
        x3 = x3()
        t1.set_downstream(x1)
        t1.set_downstream(t2)
        t1.set_downstream(x3)
        x2.set_downstream(x1)
        x2.set_downstream(t2)
        x2.set_downstream(x3)
        t3.set_downstream(x1)
        t3.set_downstream(t2)
        t3.set_downstream(x3)
    :param from_tasks: List of tasks or XComArgs to start from.
    :type from_tasks: List[airflow.models.BaseOperator] or List[airflow.models.XComArg]
    :param to_tasks: List of tasks or XComArgs to set as downstream dependencies.
    :type to_tasks: List[airflow.models.BaseOperator] or List[airflow.models.XComArg]
    """
    for task in from_tasks:
        task.set_downstream(to_tasks)
@attr.s(auto_attribs=True)
class BaseOperatorLink(metaclass=ABCMeta):