add aedp_VADX feature toggle

Summary

Adds a single toggle to enable experimental work for the Authenticated Experience Design Patterns team

Note: Optional

No impact to current site. Adding flag, to this and FE for work that will only live on staging

[ ] I fixed|updated|added unit tests and integration tests for each feature (if applicable).
[x] No error nor warning in the console.
[ ] Events are being sent to the appropriate logging solution
[ ] Documentation has been updated (link to documentation)
[x] 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 (if applicable)
[x] 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