Add basic Open API v2 support

What does this MR do?

Adds OpenAPI v2 support to DAST. The name of the API specification file and the format is passed to DAST. DAST finds this file in the /zap/wrk directory, and uses it to determine URLs available for scanning.

For the moment, this is limited to:

• Open API v2 only
• Use only with the legacy script /zap/zap-api-scan.py
• Specifications loaded via files, not URLs
• YAML specifications, not JSON
• No domain rewriting
• No authentication

This MR in part resolves issue gitlab-org/gitlab#10928 (closed).