**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
cfg_dict = conf.as_dict()
        
        assert cfg_dict['core']['unit_test_mode'] == 'True'
        assert cfg_dict['core']['percent'] == 'with%inside'
        
        assert cfg_dict['testsection']['testkey'] == '< hidden >'
        assert cfg_dict['kubernetes_environment_variables']['AIRFLOW__TESTSECTION__TESTKEY'] == '< hidden >'
    def test_conf_as_dict_source(self):
        
        cfg_dict = conf.as_dict(display_source=True)
        assert cfg_dict['core']['load_examples'][1] == 'airflow.cfg'
        assert cfg_dict['core']['load_default_connections'][1] == 'airflow.cfg'
        assert cfg_dict['testsection']['testkey'] == ('< hidden >', 'env var')
    def test_conf_as_dict_sensitive(self):
        
        cfg_dict = conf.as_dict(display_sensitive=True)
        assert cfg_dict['testsection']['testkey'] == 'testvalue'
        assert cfg_dict['testsection']['testpercent'] == 'with%percent'
        
        cfg_dict = conf.as_dict(display_sensitive=True, display_source=True)
        assert cfg_dict['testsection']['testkey'] == ('testvalue', 'env var')
    @conf_vars({("core", "percent"): "with%%inside"})
    def test_conf_as_dict_raw(self):
        
        cfg_dict = conf.as_dict(raw=True, display_sensitive=True)
        assert cfg_dict['testsection']['testkey'] == 'testvalue'
        
        assert cfg_dict['testsection']['testpercent'] == 'with%%percent'
        assert cfg_dict['core']['percent'] == 'with%%inside'
    def test_conf_as_dict_exclude_env(self):