**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.owner = owner
        self.email = email
        self.email_on_retry = email_on_retry
        self.email_on_failure = email_on_failure
        self.start_date = start_date
        if start_date and not isinstance(start_date, datetime):
            self.log.warning("start_date for %s isn't datetime.datetime", self)
        elif start_date:
            self.start_date = timezone.convert_to_utc(start_date)
        self.end_date = end_date
        if end_date:
            self.end_date = timezone.convert_to_utc(end_date)
        if trigger_rule == "dummy":
            warnings.warn(
                "dummy Trigger Rule is deprecated. Please use `TriggerRule.ALWAYS`.",
                DeprecationWarning,
                stacklevel=2,
            )
            trigger_rule = TriggerRule.ALWAYS
        if trigger_rule == "none_failed_or_skipped":
            warnings.warn(
                "none_failed_or_skipped Trigger Rule is deprecated. "
                "Please use `none_failed_min_one_success`.",
                DeprecationWarning,
                stacklevel=2,
            )
            trigger_rule = TriggerRule.NONE_FAILED_MIN_ONE_SUCCESS
        if not TriggerRule.is_valid(trigger_rule):
            raise AirflowException(
                "The trigger_rule must be one of {all_triggers},"