wasi_nn_openvino.c: implement multiple models per instance (#4380)
tested with two models:
```
--load-graph=id=graph1,file=public/license-plate-recognition-barrier-0007/FP32/license-plate-recognition-barrier-0007.xml,file=public/license-plate-recognition-barrier-0007/FP32/license-plate-recognition-barrier-0007.bin \
--load-graph=id=graph2,file=classify/model.xml,file=classify/model.bin \
--init-execution-context=id=exec1,graph-id=graph1 \
--init-execution-context=id=exec2,graph-id=graph2 \
--set-input=context-id=exec1,dim=1,dim=24,dim=94,dim=3,file=out.bin \
--set-input=context-id=exec2,file=classify/banana-3x224x224-bgr.bin,dim=1,dim=3,dim=224,dim=224 \
--compute=context-id=exec1 \
--compute=context-id=exec2 \
--get-output=context-id=exec1,file=exec1-result.bin \
--get-output=context-id=exec2,file=exec2-result.bin
```
a detailed HOWTO: https://github.com/bytecodealliance/wasm-micro-runtime/pull/4380#issuecomment-2986882718
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