Vendor non-Python dependencies in `third_party/`

[adeebshihadeh]

Read this for rationale: https://htmx.org/essays/vendoring/

• ffmpeg (first attempt here got reverted minimal ffmpeg build #36138)
• capnproto
• Qt5 Rewrite ui in Raylib (from Qt) #33301
• gcc-arm-none-eabi (lots of space to save here)
• opencl
• ZMQ
• eigen (maybe we can just remove after Rewrite ui in Raylib (from Qt) #33301?)

---

Once this is done, our setup scripts should be able to take a fresh Ubuntu or macOS and get setup with a simple uv sync.