wasi-nn: Add support of wasi-nn as shared lib (#2310)

## Context

Currently, WAMR supports compiling iwasm with flag `WAMR_BUILD_WASI_NN`.
However, there are scenarios where the user might prefer having it as a shared library.

## Proposed Changes

Decouple wasi-nn context management by internally managing the context given
a module instance reference.

core/iwasm/interpreter/wasm_runtime.h

@@ -241,10 +241,6 @@ typedef struct WASMModuleInstanceExtra {
         && WASM_ENABLE_LAZY_JIT != 0)
     WASMModuleInstance *next;
 #endif
-
-#if WASM_ENABLE_WASI_NN != 0
-    WASINNContext *wasi_nn_ctx;
-#endif
 } WASMModuleInstanceExtra;
 
 struct AOTFuncPerfProfInfo;