create storage subsystem for OAuth2 tokens

[mbjones]

We need to securely store:

• access token and its expiration datetime
• refresh token and its expiration datetime

We may also want to store and/or use:

• identity claims about the user (e.g., username, email, scopes, etc)

Consider using the keyring package for the token storage, see https://keyring.r-lib.org/reference/key_get.html

This could all be potentially be wrapped into a new Session class, which might have methods like login(), logout(), set_access_token(), get_access_token(), set_refresh_token(), get_refresh_token(), refresh_session(), and maybe others.