Make DAST more easily extendable

What does this MR do and why?

To allow DAST users to more easily create their own custom DAST jobs, this MR moves some of the DAST configurations to a reusable section name .dast_base in both the current dast template DAST.gitlab-ci.yml and test latest version DAST.latest.gitlab-ci.yml

These changes should have no effect on how the DAST template is currently used.

Related Issue

#340420

Manual QA

Template

To allow the DAST scan to run for both of these templates, I've added the dast stage and a target website to variables. These would normally be added after the user includes the template.

stages:
    - dast

variables:
  DAST_WEBSITE: https://google-gruyere.appspot.com/564959828417281418006207909483102970029/`

DAST.gitlab-ci.yml

• project
• pipeline
• job

DAST.latest.gitlab-ci.yml

• project
• pipeline
• job