Semgrep Service: Add rudimentary authentication

What does this MR do?

Add basic authentication against the GitLab API to the semgrep service. For our purposes it is sufficient to perform the authentication against the gitlab project https://gitlab.com/gitlab-org/secure/sast-ide-integration which is the reason why the project id is hardcoded.

What are the relevant issue numbers?

Related MR: !421 (closed) Related Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/462808

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