**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
with tempfile.NamedTemporaryFile(delete=False) as cmd_file:
            cmd_file.write(b"#!/usr/bin/env bash\n")
            cmd_file.write(b"echo -n difficult_unpredictable_cat_password\n")
            cmd_file.flush()
            os.chmod(cmd_file.name, 0o0555)
            cmd_file.close()
            with mock.patch.dict("os.environ", {"AIRFLOW__WEBSERVER__SECRET_KEY_CMD": cmd_file.name}):
                content = conf.getsection("webserver")
            os.unlink(cmd_file.name)
        assert content["secret_key"] == "difficult_unpredictable_cat_password"
    def test_kubernetes_environment_variables_section(self):