**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
self.on_retry_callback = on_retry_callback
        self._pre_execute_hook = pre_execute
        self._post_execute_hook = post_execute
        if isinstance(retry_delay, timedelta):
            self.retry_delay = retry_delay
        else:
            self.log.debug("Retry_delay isn't timedelta object, assuming secs")
            self.retry_delay = timedelta(seconds=retry_delay)
        self.retry_exponential_backoff = retry_exponential_backoff
        self.max_retry_delay = max_retry_delay
        if max_retry_delay:
            if isinstance(max_retry_delay, timedelta):
                self.max_retry_delay = max_retry_delay
            else:
                self.log.debug("max_retry_delay isn't a timedelta object, assuming secs")
                self.max_retry_delay = timedelta(seconds=max_retry_delay)
        self.params = ParamsDict(params)
        if priority_weight is not None and not isinstance(priority_weight, int):
            raise AirflowException(
                f"`priority_weight` for task '{self.task_id}' only accepts integers, "
                f"received '{type(priority_weight)}'."
            )
        self.priority_weight = priority_weight
        if not WeightRule.is_valid(weight_rule):
            raise AirflowException(
                "The weight_rule must be one of {all_weight_rules},"
                "'{d}.{t}'; received '{tr}'.".format(
                    all_weight_rules=WeightRule.all_weight_rules,
                    d=dag.dag_id if dag else "",
                    t=task_id,