Needs to use new /token/sign endpoint of mod-at

Currently there is a dependency on the legacy /token endpoint of mod-at which is part of mod-users-bl's password reset process.

The needed change should have the following logic

• Attempt to get the token from `/token/sign`

• If 404, fallback to `/token`

Acceptance criteria:

• Change code in AuthTokenClientImpl.java

• Updated unit tests to cover both scenarios