**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
:param run_as_user: unix username to impersonate while running the task
    :type run_as_user: str
    :param max_active_tis_per_dag: When set, a task will be able to limit the concurrent
        runs across execution_dates.
    :type max_active_tis_per_dag: int
    :param executor_config: Additional task-level configuration parameters that are
        interpreted by a specific executor. Parameters are namespaced by the name of
        executor.
        **Example**: to run this task in a specific docker container through
        the KubernetesExecutor ::
            MyOperator(...,
                executor_config={
                    "KubernetesExecutor":
                        {"image": "myCustomDockerImage"}
                }
            )
    :type executor_config: dict
    :param do_xcom_push: if True, an XCom is pushed containing the Operator's
        result
    :type do_xcom_push: bool
    :param task_group: The TaskGroup to which the task should belong. This is typically provided when not
        using a TaskGroup as a context manager.
    :type task_group: airflow.utils.task_group.TaskGroup
    :param doc: Add documentation or notes to your Task objects that is visible in
        Task Instance details View in the Webserver
    :type doc: str
    :param doc_md: Add documentation (in Markdown format) or notes to your Task objects
        that is visible in Task Instance details View in the Webserver
    :type doc_md: str
    :param doc_rst: Add documentation (in RST format) or notes to your Task objects