SAST scanner service: add oauth handling

What does this MR do?

This MR adds support for oauth tokens to the SAST scanner service.

What are the relevant issue numbers?

Authentication and Authorization for real-time ... (gitlab-org/gitlab#467934 - closed) • Julian Thome, Jason Leasure • Next 1-3 releases

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 updated/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