Add GitHub Action to enforce Apache 2.0 license headers

[lornakelly]

Summary

Add a GitHub Actions workflow that validates Apache 2.0 license headers are present in all applicable source files and fails the build if headers are missing. The check must execute on every Pull Request.

Goals

Ensure all source files comply with Apache 2.0 licensing requirements.

• Prevent merging code without proper license headers.
• Automate enforcement instead of relying on manual review.
• Maintain CNCF/open-source governance standards.

Non-Goals

• Complex legal compliance tooling beyond header validation.
• Checking third-party dependency licenses.