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/aot/aot_runtime.h

@@ -89,9 +89,6 @@ typedef struct AOTFunctionInstance {
 
 typedef struct AOTModuleInstanceExtra {
     CApiFuncImport *c_api_func_imports;
-#if WASM_ENABLE_WASI_NN != 0
-    WASINNContext *wasi_nn_ctx;
-#endif
 } AOTModuleInstanceExtra;
 
 #if defined(OS_ENABLE_HW_BOUND_CHECK) && defined(BH_PLATFORM_WINDOWS)