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/libraries/wasi-nn/external/README.md

@@ -0,0 +1,13 @@
+# wasi-nn as shared library
+
+Example on how to create libwasi-nn (external library) instead of embedding wasi-nn inside iwasm
+
+From folder `core/iwasm/libraries/wasi-nn/test`, build the test and run
+
+```sh
+../external/build/iwasm \
+    --dir=. \
+    --env="TARGET=cpu" \
+    --native-lib=../external/build/libwasi-nn.so \
+    test_tensorflow.wasm 
+```