Send via header with all DAST requests

What does this MR do?

This MR forces ZAP to add the following header to all requests:

Via: GitLab DAST/ZAP v1.50.0 (where 1.50.0 is the current DAST version)

When a Browserker scan is run, Browserker also adds a Via header. Due to a limitation of the ZAP replacer plugin, this is unfortunately overwritten by this header. To help engineers/users understand which system made a request, DAST configures Browserker to add the following header to requests it sends:

Via-Scanner: Browserker

What are the relevant issue numbers?

gitlab-org/gitlab#327564 (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