Add vendored CI/CD templates for DAST job

What does this MR do?

Adds a vendored template for the DAST job that can be referred from .gitlab-ci.yml config using

include:
  template: DAST.gitlab-ci.yml

syntax.

What are the relevant issue numbers?

#8663 (closed)

Corresponding CE MR

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25614

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• [ ] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [ ] Tested in all supported browsers
• Conforms to the code review guidelines
• [ ] Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer