Move tokens to state instead of config

Problem/Motivation

The module currently stores the client secret as well as access and refresh token in a config. This should be moved to state or support drupal/key.

Proposed resolution

- Move storage for secret and tokens to state or support drupal/key.

Remaining tasks

- Define storage in state
- Add support for support drupal/key.
- Add tests.

User interface changes

API changes

Data model changes