Move DAST basic auth option behind feature flag

What does this MR do and why?

This moves the !96514 (merged) changes behind a feature flag - dast_api_scanner.

It was found that the backend is not quite ready, we need to roll this out with other API security changes.

Screenshots or screen recordings

With FF disabled => no auth option	With FF enabled => basic auth option

How to set up and validate locally

DAST Site Profile - http://gdk.test:3000/flightjs/Flight/-/security/configuration/profile_library/dast_site_profiles/new

MR acceptance checklist

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