Change the ZAP hook used to authenticate sessions

What does this MR do?

When ZAProxy starts, DAST listens to the zap_access_target hook. When the hook is called, DAST creates a new session, and if required, starts a browser and logs the user in using supplied credentials.

This MR changes this behaviour to instead listen for the zap_started hook. The primary reason for this is that the API scanning code does not have a zap_access_target hook, therefore any functionality we add to that hook would not work for an API scan.

This in part resolves issue gitlab-org/gitlab#10928 (closed).

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer