Implement pre scan verification mode for DAST Analyzer

The On Demand API Scan feature allows running DAST scans directly from the GitLab UX. When performing such a scan a user creates site profile with a number of configuration options that includes authentication, target URL, etc. To improve the user experience, a new validation feature is being added. This will allow users to quickly check if their configuration works without running a full scan.

To support this work, DAST needs the ability to perform a verification process and report back success or failure to an internal API.

Proposal

TBC