**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
def make_config():
            test_conf = AirflowConfigParser(default_config='')
            
            
            test_conf.deprecated_values = {
                'core': {'hostname_callable': (re.compile(r':'), r'.', '2.1')},
            }
            test_conf.read_dict(conf_dict)
            test_conf.validate()
            return test_conf
        with pytest.warns(FutureWarning):
            with unittest.mock.patch.dict('os.environ', AIRFLOW__CORE__HOSTNAME_CALLABLE='socket:getfqdn'):
                test_conf = make_config()
                assert test_conf.get('core', 'hostname_callable') == 'socket.getfqdn'
        with reset_warning_registry():
            with warnings.catch_warnings(record=True) as warning:
                with unittest.mock.patch.dict(
                    'os.environ',
                    AIRFLOW__CORE__HOSTNAME_CALLABLE='CarrierPigeon',
                ):
                    test_conf = make_config()
                    assert test_conf.get('core', 'hostname_callable') == 'CarrierPigeon'
                    assert [] == warning
    def test_deprecated_funcs(self):