**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
config = textwrap.dedent(
            """
            [core]
            dags_folder = {AIRFLOW_HOME}/dags
            sql_alchemy_conn = sqlite:///{AIRFLOW_HOME}/airflow.db
            parallelism = 32
            fernet_key = {FERNET_KEY}
        """
        )
        cfg = parameterized_config(config)
        
        assert "[core]" in cfg
        assert "dags_folder" in cfg
        assert "sql_alchemy_conn" in cfg
        assert "fernet_key" in cfg
        
        assert "{AIRFLOW_HOME}" not in cfg
        assert "{FERNET_KEY}" not in cfg
    def test_config_use_original_when_original_and_fallback_are_present(self):