Integrate WASI-NN into WAMR (#1521)

Initial integration of WASI-NN based on #1225:
- Implement the library core/iwasm/libraries/wasi-nn
- Support TensorFlow, CPU, F32 at the first stage
- Add cmake variable `-DWAMR_BUILD_WASI_NN`
- Add test case based on Docker image and update document

Refer to #1573

core/deps/install_tensorflow.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+DEPS_ROOT=$(cd "$(dirname "$0")/" && pwd)
+cd ${DEPS_ROOT}
+
+echo "Downloading tensorflow in ${PWD}..."
+
+git clone https://github.com/tensorflow/tensorflow.git tensorflow-src \
+    --branch v2.9.2
+
+exit 0