text: Title: Config.load_env type should explicitly allow strings
  Body: https://github.com/sanic-org/sanic/blob/main/sanic/config.py#L44
  
  ```python
          if load_env:
              prefix = SANIC_PREFIX if load_env is True else load_env
  ```
  
  `load_env` defaults to `True`, but you can pass a string if you want a different prefix. We should make this type consistent by either explicitly accepting both a `bool` and ` `str`, or (perhaps a more elegant solution) is just to ONLY accept a string and default it to `SANIC_PREFIX` since this is what it is trying to do.