Refactor WASI-NN to simplify the support for multiple frameworks (#1834)

- Reorganize the library structure
- Use the latest version of `wasi-nn` wit (Oct 25, 2022):
    https://github.com/WebAssembly/wasi-nn/blob/0f77c48ec195748990ff67928a4b3eef5f16c2de/wasi-nn.wit.md
- Split logic that converts WASM structs to native structs in a separate file
- Simplify addition of new frameworks

build-scripts/runtime_lib.cmake

@@ -96,9 +96,13 @@ if (WAMR_BUILD_LIB_PTHREAD_SEMAPHORE EQUAL 1)
 endif ()
 
 if (WAMR_BUILD_WASI_NN EQUAL 1)
-    execute_process(COMMAND ${WAMR_ROOT_DIR}/core/deps/install_tensorflow.sh
-                    RESULT_VARIABLE TENSORFLOW_RESULT
-    )
+    if (NOT EXISTS "${WAMR_ROOT_DIR}/core/deps/tensorflow-src")
+        execute_process(COMMAND ${WAMR_ROOT_DIR}/core/deps/install_tensorflow.sh
+                        RESULT_VARIABLE TENSORFLOW_RESULT
+        )
+    else ()
+        message("Tensorflow is already downloaded.")
+    endif()
     set(TENSORFLOW_SOURCE_DIR "${WAMR_ROOT_DIR}/core/deps/tensorflow-src")
     include_directories (${CMAKE_CURRENT_BINARY_DIR}/flatbuffers/include)
     include_directories (${TENSORFLOW_SOURCE_DIR})