Support loading secrets from files

### What feature is it?

Inside the environment variables, it should be possible to load secrets from files defined by a macro like this:
`MY_API_KEY=%{file:/etc/secrets/api_secret}%`

### What problem does this feature solve?

It would enable upstream distributions like NixOS to use techniques like systemds LoadCredentials

### Additional description

_No response_

### This is not a duplicated feature request or new RSS proposal

- [x] I have searched [existing issues](https://github.com/DIYgod/RSSHub/issues) to ensure this feature has not already been requested and this is not a [new RSS proposal](https://github.com/DIYgod/RSSHub/issues/new?assignees=&labels=RSS+proposal&template=rss_request_en.yml).