**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
"""Get Config Value from a Secret Backend"""
        mock_client = mock.MagicMock()
        mock_hvac.Client.return_value = mock_client
        mock_client.secrets.kv.v2.read_secret_version.return_value = {
            'request_id': '2d48a2ad-6bcb-e5b6-429d-da35fdf31f56',
            'lease_id': '',
            'renewable': False,
            'lease_duration': 0,
            'data': {
                'data': {'value': 'sqlite:////Users/airflow/airflow/airflow.db'},
                'metadata': {