Adds DAST_SPIDER_START_AT_HOST env var

What does this MR do?

Adds DAST_SPIDER_START_AT_HOST, which allows the user to decide whether the scan should start at the target or at the target's host. It defaults to true to preserve the existing functionality

It will be set to default to false in %14.0: gitlab-org/gitlab#267403 (closed)

What are the relevant issue numbers?

gitlab-org/gitlab#258805 (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