Use SAST scan API endpoint in IDE extension

Problem to solve

To perform security scans, the extension will use a GitLab API REST endpoint.

Currently, the remote security scan feature in gitlab-vscode-extension and gitlab-lsp are configured with a serviceUrl for testing. That URL can be eliminated and instead, the GitlabAPIClient interface can be amended.

Proposal

• add an instance of GitlabAPIClient to DefaultSecurityDiagnosticsPublisher
• change the fetch in security_diagnostics_publisher.ts to a fetchFromApi
  • reformat POST body as JSON instead of multipart-mime
• remove serviceUrl configuration item
  • from gitlab-lsp
  • from gitlab-vscode-extension
• update doc in gitlab-lsp