Refresh access token

A renewed access token that can be attached to the header of any API request. Refreshing does not reset the session: it only issues a new token pair within the session's remaining lifetime, which is capped to 12 hours. As the session nears that limit, expiresIn and refreshExpiresIn shrink to whatever time is left, and can eventually reach zero even though the refresh token hasn't expired. A shrinking refreshExpiresIn across repeated refreshes is a sign the session is ending: call /auth-api/v2/authenticate again to start a new session, rather than refreshing more often.