Add alternate global route for vaos module

[drewconnelly]

The current vets-api vaos module sends all requests to a single vaos-service endpoint. In support of potential future testing efforts, add an optional feature flag (defaulted off) that will redirect the VAOS requests to an alternate URL.

[devin-mccurdy]

PRs:

• https://github.com/department-of-veterans-affairs/vets-api/pull/18753
• https://github.com/department-of-veterans-affairs/vets-api/pull/18755