**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
if self.airflow_defaults.has_option(section, key) or 'fallback' in kwargs:
            return expand_env_var(self.airflow_defaults.get(section, key, **kwargs))
        else:
            log.warning("section/key [%s/%s] not found in config", section, key)
            raise AirflowConfigException(f"section/key [{section}/{key}] not found in config")
    def _get_option_from_secrets(self, deprecated_key, deprecated_section, key, section):
        
        option = self._get_secret_option(section, key)
        if option:
            return option
        if deprecated_section:
            option = self._get_secret_option(deprecated_section, deprecated_key)
            if option:
                self._warn_deprecate(section, key, deprecated_section, deprecated_key)
                return option
        return None
    def _get_option_from_commands(self, deprecated_key, deprecated_section, key, section):