Implement UX to enable SLSA provenance generation for build artifacts

As discussed in #547903 (comment 2748278333), the purpose of this issue is to implement the backend controls to support the UX to enable SLSA provenance generation for build artifacts.

This change will modify Ci::BuildFinishedWorker#should_publish_provenance to return true when the following conditions are met:

• ATTEST_BUILD_ARTIFACTS = true
• stage = build
• artifacts exist
• project.public? is true