Add support for custom builders

What does this MR do?

This PR add support for custom builders.
Builders are defined in https://gitlab.com/feistel/gemnasium/-/tree/master/builder and registered in the init function in their own package. (Remember to add an import in main.go if you add a custom builder or the function won't be called)

What are the relevant issue numbers?

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
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer