DAST Site validation - Add DastSiteValidationWorker - Backend

• overview: &2912 (closed)
• parent issue: #233020 (closed)
• context

Summary

this issue involves creation of a background job that validates a url using a dast_site_token.

• receives dast_site_validation_id
• looks up dast_site_validation and dast_site_validation_token in the db
• makes an http request to url and looks for token
• if success update state fields and terminate otherwise update state fields and retry until retry limit

see @appelt's comment about using lib/gitlab/url_blocker.rb to validate the url and ensure we don't show the response to users anywhere.

Implementation Plan

• create new worker called DastSiteValidationWorker