Dependency Scanning custom images support

Following #5232 (closed), and similar to https://gitlab.com/gitlab-org/gitlab-ee/issues/5773, we could let the users use their own, custom docker images for Dependency Scanning. For example, some users have issues with the current Dependency Scanning image, as it requires docker:dind to work correctly. Therefore, they are running directly the new Java Maven image, by naming the job dependency_scanning. While this is working fine, it's limiting the options to only one job (Java Maven in this case).

This issue is to allow users to create dependency_scanning:<custom job name> jobs.

Requirements:

• The job name is a convention, and must start with :
• The job must create an artifact named gl-dependency-scanning-report.json

Tasks:

• Specify the format for gl-dependency-scanning-report.json
• Update backend to aggregate all artifacts in one report (this is transparent for FrontEnd).

/cc @bikebilly