Add va_online_scheduling_enable_OH_slots_search feature flag

[devin-mccurdy]

Summary

• This work is behind a feature toggle (flipper): NO
• This PR adds a feature flag that will route appointment slot search operations to the new VPG service if enabled. If it is disabled appointment slot search operations will continue to use vaos-service.
• Unified Appointment Experience team
• The success criteria being targeted is the ability to support appointment operations at Oracle Health sites by using VPG for searching appointment slots instead of vaos-service.

Related issue(s)

department-of-veterans-affairs/va.gov-team#76083

Testing done

• Confirmed that the new feature flag is displayed when running locally at http://localhost:3000/flipper/features

What areas of the site does it impact?

This change only impacts flipper

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

[drewconnelly]

LGTM