CI Pipeline Improvements for Cross-Platform Support

[tustanivsky]

Current State

The existing CI pipeline supports Windows and Linux platforms using pre-built Docker images:

• chore: Build Windows UE Docker images in CI #792
• chore: Build custom UE Linux Docker images to update CI #864

This Docker-based approach works well for these platforms but creates significant limitations for expanding current platform coverage.

Problem Statement

The current Docker-based CI approach cannot scale to support additional critical platforms:

• macOS/iOS: No official macOS Docker images exist, making dockerized macOS Unreal Engine builds problematic
• Android: Current unreal-docker tooling doesn't provide mechanisms for introducing additional platform support

Proposed Solution

Transition to custom engine builds with cloud-based distribution:

• Build Unreal Engine from source in CI for each target platform
• Upload built engines as ZIP archives to cloud storage (e.g., GCS bucket)
• Other CI workflows can pull these pre-built engines for packaging and testing

This approach has proven feasible in similar contexts:

• Distributing installed build of the engine for Sentry's console plugin extensions on a dedicated CI build server
• Cesium Unreal uses this same pattern effectively

Expected Outcomes

• Full Platform Coverage: Enable CI support for macOS, iOS, and Android platforms
• Scalable Architecture: Easily add new platforms without Docker limitations
• Improved Test Coverage: Comprehensive cross-platform testing capabilities
• Reduced Maintenance: Eliminate Docker-specific workarounds and limitations

Related issues

• Integration/smoke test #77
• Improve CI - test on more versions, install final package for validation, test on more linux distros #575
• Add automation to test Android e2e #964