Make wasi-nn backends as separated shared libraries (#3509)

- All files under *core/iwasm/libraries/wasi-nn* are compiled as shared libraries
- *wasi-nn.c* is shared between backends
- Every backend has a separated shared library
- If wasi-nn feature is enabled, iwasm will depend on shared library libiwasm.so
  instead of linking static library libvmlib.a

core/iwasm/libraries/wasi-nn/test/utils.h

@@ -8,7 +8,7 @@
 
 #include <stdint.h>
 
-#include "wasi_nn.h"
+#include "wasi_nn_types.h"
 
 #define MAX_MODEL_SIZE 85000000
 #define MAX_OUTPUT_TENSOR_SIZE 1000000