**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
key = "AIRFLOW__CORE__FERNET_KEY"
        value = "some value"
        with mock.patch.dict('os.environ', {key: value}):
            fernet_key = conf.get('core', 'FERNET_KEY')
        assert value == fernet_key
    def test_config_override_original_when_empty_envvar_is_provided(self):
        key = "AIRFLOW__CORE__FERNET_KEY"
        value = "some value"
        with mock.patch.dict('os.environ', {key: value}):
            fernet_key = conf.get('core', 'FERNET_KEY')
        assert value == fernet_key
    @mock.patch.dict("os.environ", {"AIRFLOW__CORE__DAGS_FOLDER": "/tmp/test_folder"})
    def test_write_should_respect_env_variable(self):
        with io.StringIO() as string_file:
            conf.write(string_file)
            content = string_file.getvalue()
        assert "dags_folder = /tmp/test_folder" in content
    def test_run_command(self):
        write = r'sys.stdout.buffer.write("\u1000foo".encode("utf8"))'
        cmd = f'import sys; {write}; sys.stdout.flush()'
        assert run_command(f"python -c '{cmd}'") == '\u1000foo'
        assert run_command('echo "foo bar"') == 'foo bar\n'
        with pytest.raises(AirflowConfigException):
            run_command('bash -c "exit 1"')
    def test_confirm_unittest_mod(self):
        assert conf.get('core', 'unit_test_mode')