**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
"""Returns reference to XCom pushed by current operator"""
        from airflow.models.xcom_arg import XComArg
        return XComArg(operator=self)
    @staticmethod
    def xcom_push(
        context: Any,
        key: str,
        value: Any,
        execution_date: Optional[datetime] = None,
    ) -> None:
        """
        Make an XCom available for tasks to pull.
        :param context: Execution Context Dictionary
        :type: Any
        :param key: A key for the XCom
        :type key: str
        :param value: A value for the XCom. The value is pickled and stored
            in the database.
        :type value: any pickleable object
        :param execution_date: if provided, the XCom will not be visible until
            this date. This can be used, for example, to send a message to a
            task on a future date without it being immediately visible.
        :type execution_date: datetime
        """
        context['ti'].xcom_push(key=key, value=value, execution_date=execution_date)
    @staticmethod
    def xcom_pull(