**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/configuration.py
val = self.get(section, key, **kwargs)
        try:
            return int(val)
        except ValueError:
            raise AirflowConfigException(
                f'Failed to convert value to int. Please check "{key}" key in "{section}" section. '
                f'Current value: "{val}".'
            )
    def getfloat(self, section, key, **kwargs):
        val = self.get(section, key, **kwargs)
        try:
            return float(val)
        except ValueError:
            raise AirflowConfigException(
                f'Failed to convert value to float. Please check "{key}" key in "{section}" section. '
                f'Current value: "{val}".'
            )
    def getimport(self, section, key, **kwargs):