Add vendored CI/CD templates for DAST job for EE

What does this MR do?

Adds a vendored template for the DAST job that can be referred from .gitlab-ci.yml using include directive and some code to support this functionality in EE.

It was decided that it's acceptable to keep this template in CE as this has to licensing issues and reduces the code complexity and amount of work to implement this feature in EE.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8663

Corresponding EE MR

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9752

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.~~
• Security reports checked/validated by reviewer