**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
from tests.test_utils.config import conf_vars
from tests.test_utils.reset_warning_registry import reset_warning_registry
@unittest.mock.patch.dict(
    'os.environ',
    {
        'AIRFLOW__TESTSECTION__TESTKEY': 'testvalue',
        'AIRFLOW__TESTSECTION__TESTPERCENT': 'with%percent',
        'AIRFLOW__TESTCMDENV__ITSACOMMAND_CMD': 'echo -n "OK"',
        'AIRFLOW__TESTCMDENV__NOTACOMMAND_CMD': 'echo -n "NOT OK"',
    },
)
class TestConf:
    def test_airflow_home_default(self):
        with unittest.mock.patch.dict('os.environ'):
            if 'AIRFLOW_HOME' in os.environ:
                del os.environ['AIRFLOW_HOME']
            assert get_airflow_home() == expand_env_var('~/airflow')
    def test_airflow_home_override(self):
        with unittest.mock.patch.dict('os.environ', AIRFLOW_HOME='/path/to/airflow'):
            assert get_airflow_home() == '/path/to/airflow'
    def test_airflow_config_default(self):
        with unittest.mock.patch.dict('os.environ'):
            if 'AIRFLOW_CONFIG' in os.environ:
                del os.environ['AIRFLOW_CONFIG']
            assert get_airflow_config('/home/airflow') == expand_env_var('/home/airflow/airflow.cfg')
    def test_airflow_config_override(self):
        with unittest.mock.patch.dict('os.environ', AIRFLOW_CONFIG='/path/to/airflow/airflow.cfg'):
            assert get_airflow_config('/home//airflow') == '/path/to/airflow/airflow.cfg'
    @conf_vars({("core", "percent"): "with%%inside"})
    def test_case_sensitivity(self):