Add OpenVINO backend for Intel iGPU/dGPU (Arc)

Mirrors the rocm/cuda pattern: ort::ep::OpenVINO with device_type "GPU"
(covers both Intel integrated graphics and Arc discrete GPUs through the
same EP), load-dynamic/dlopen so no OpenVINO toolkit is needed at build
time, and its own --openvino flag / backend = "openvino" config value.
Folded into the `full` feature alongside npu/rocm/cuda.

OpenVINO's provider options go through a generic key/value FFI interface
rather than a fixed C struct (unlike MIGraphX's OrtMIGraphXProviderOptions),
so it should be less exposed to the ABI-version-skew crash MIGraphX hit --
but that's inference from the EP's design, not verified against real
hardware. Like CUDA, this is compile-checked only: no Intel GPU in this
dev environment to runtime-verify against.

Version bump: breadmill 0.2.2 -> 0.2.3.

packaging/breadmill.service

@@ -8,7 +8,8 @@ Type=simple
 # Required (not just a perf tweak) when backend = "rocm": without a valid
 # cache dir, ONNX Runtime's MIGraphX EP reads an uninitialized cache path and
 # crashes on the first query instead of just recompiling every restart. Inert
-# for cpu/npu/cuda backends. See README.md#gpu-backend-notes.
+# for cpu/npu/cuda/openvino backends (openvino's cache dir is set in code,
+# not via env var). See README.md#gpu-backend-notes.
 Environment=ORT_MIGRAPHX_MODEL_CACHE_PATH=%h/.cache/breadsearch/migraphx-cache
 ExecStart=%h/.cargo/bin/breadmill
 Restart=on-failure