**Description**

Enabled list of cors headers origin allowed

**Use case / motivation**

I'm currently developping Django application working with apache airflow for some external work through api. From Django, user run dag and a js request wait for dag end. By this way, I enabled cors headers in airflow.cfg with correct url. But now, I've  environment (dev & prod). It could be useful to be able to set more than one url in airflow.cfg cors header origin allowed

Thank's in advance :) 

airflow/configuration.py
#
#
import functools
import json
import logging
import multiprocessing
import os
import pathlib
import re
import shlex
import subprocess
import sys
import warnings
from base64 import b64encode
from collections import OrderedDict
from configparser import _UNSET, ConfigParser, NoOptionError, NoSectionError  
from json.decoder import JSONDecodeError
from typing import Dict, List, Optional, Union
from airflow.exceptions import AirflowConfigException
from airflow.secrets import DEFAULT_SECRETS_SEARCH_PATH, BaseSecretsBackend
from airflow.utils import yaml
from airflow.utils.module_loading import import_string
log = logging.getLogger(__name__)
if not sys.warnoptions:
    warnings.filterwarnings(action='default', category=DeprecationWarning, module='airflow')
    warnings.filterwarnings(action='default', category=PendingDeprecationWarning, module='airflow')
def expand_env_var(env_var):