**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

airflow/configuration.py
return f'{self.ENV_VAR_PREFIX}{section.upper()}__{key.upper()}'
    def _get_env_var_option(self, section, key):
        
        env_var = self._env_var_name(section, key)
        if env_var in os.environ:
            return expand_env_var(os.environ[env_var])
        
        env_var_cmd = env_var + '_CMD'
        if env_var_cmd in os.environ:
            
            if (section, key) in self.sensitive_config_values:
                return run_command(os.environ[env_var_cmd])
        
        env_var_secret_path = env_var + '_SECRET'
        if env_var_secret_path in os.environ:
            
            if (section, key) in self.sensitive_config_values:
                return _get_config_value_from_secret_backend(os.environ[env_var_secret_path])
        return None
    def _get_cmd_option(self, section, key):
        fallback_key = key + '_cmd'
        
        if (section, key) in self.sensitive_config_values:
            if super().has_option(section, fallback_key):
                command = super().get(section, fallback_key)
                return run_command(command)
        return None
    def _get_secret_option(self, section, key):