Add retry handling for .login() method

[bltravis]

When using a FolioClient instance to manage auth tokens for a long-running script, temporary server errors (502, 503, 504) can cause an exception. It should be possible for FolioClient to support (optional) retries of the authn flow. The number of retries, the backoff factor, and the initial retry delay should all be configurable (by environment variables).