**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

tests/core/test_configuration.py
for func in [
            'load_test_config',
            'get',
            'getboolean',
            'getfloat',
            'getint',
            'has_option',
            'remove_option',
            'as_dict',
            'set',
        ]:
            with mock.patch(f'airflow.configuration.conf.{func}') as mock_method:
                with pytest.warns(DeprecationWarning):
                    getattr(configuration, func)()
                mock_method.assert_called_once()
    def test_command_from_env(self):