Add support for exporting config path to JSON key file

Uploading patch file of the described change.

The json_path config works.

Didn't test the old json fid way, and I think it should be fully removed.

Also changing return FALSE to throw \Exception is not the best idea. Because if the json_path is not set (for example, on the local environment as opposed to production), on the Status report page you always see an error message, which doesn't even tell which authentication produces the error.

So probably needs work (but I'm not the module maintainer, so won't change the issue status).