**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
key = env_var.replace(section_prefix, '')
                if key.endswith("_CMD"):
                    key = key[:-4]
                key = key.lower()
                _section[key] = self._get_env_var_option(section, key)
        for key, val in _section.items():
            try:
                val = int(val)
            except ValueError:
                try:
                    val = float(val)
                except ValueError:
                    if val.lower() in ('t', 'true'):
                        val = True
                    elif val.lower() in ('f', 'false'):
                        val = False
            _section[key] = val
        return _section
    def write(self, fp, space_around_delimiters=True):
        
        
        if space_around_delimiters:
            delimiter = f" {self._delimiters[0]} "
        else:
            delimiter = self._delimiters[0]
        if self._defaults:
            self._write_section(fp, self.default_section, self._defaults.items(), delimiter)
        for section in self._sections:
            self._write_section(fp, section, self.getsection(section).items(), delimiter)
    def as_dict(