Changes

The response from /oauth/token includes an expires_in field, which is the time (from issuance) that the access token expires in. As noted in #356, customers wishing to know the expiry status of the token need to manage this manually, by calculating the expiry time from the issuance and expires_in value. This change introduces a new calculated value to TokenHolder that represents the expiration time of the access token.

Reviewer notes

• See the individual commit history to see how tests and refactorings were introduced to mitigate any unintended side effects (added tests for regression cases, refactored to custom deserializer, then added new calculated field)
• Date type was used for the new expiresAt field, to provide consistency with other types (customers can use Date.from(Instant value) to take advantage of modern date/time APIs, as the unit tests demonstrate)