LLVM: introduce blender testing

[kasaurov]

Motivation

The PR introduces infrastructure to run blender tests to be on par with existing PSDB testing.

N.B. The PR isn't yet ready, the place to get the scenes & reference files from is still to be decided.

Technical Details

New files introduced in external-builds/blender/:

• run_blender_tests.py -- test runner which invokes Blender Cycles with the HIP, renders scenes from a manifest, performs inline SSIM/MSE image comparison against optional reference images, captures full Blender output per scene, extracts render summaries (GPU, samples, timings), and produces a pass/fail report.
• requirements.txt -- Python dependencies for image comparison (opencv-python-headless, scikit-image, numpy).
• scenes.txt -- Manifest listing scene files and frame numbers to render.
• README.md -- Documentation.

New workflow .github/workflows/test_blender.yml:

• Reusable workflow_call workflow for running Blender HIP rendering tests in CI.
• Downloads Blender and scene archives from configurable URLs.
• Runs scenes and validates the results
• Uploads rendered images and per-scene logs as CI artifacts.

Test Plan

Local testing of Python script in the Docker

Test Result

As expected

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.