Implement module instance context APIs (#2436)

Introduce module instance context APIs which can set one or more contexts created
by the embedder for a wasm module instance:
```C
    wasm_runtime_create_context_key
    wasm_runtime_destroy_context_key
    wasm_runtime_set_context
    wasm_runtime_set_context_spread
    wasm_runtime_get_context
```

And make libc-wasi use it and set wasi context as the first context bound to the wasm
module instance.

Also add samples.

Refer to https://github.com/bytecodealliance/wasm-micro-runtime/issues/2460.

samples/inst-context-threads/build.sh

@@ -0,0 +1,61 @@
+#
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+
+#!/bin/bash
+
+CURR_DIR=$PWD
+WAMR_DIR=${PWD}/../..
+OUT_DIR=${PWD}/out
+
+WASM_APPS=${PWD}/wasm-apps
+
+
+rm -rf ${OUT_DIR}
+mkdir ${OUT_DIR}
+mkdir ${OUT_DIR}/wasm-apps
+
+
+echo "#####################build inst-context project"
+cd ${CURR_DIR}
+mkdir -p cmake_build
+cd cmake_build
+cmake ..
+make -j ${nproc}
+if [ $? != 0 ];then
+    echo "BUILD_FAIL inst-context exit as $?\n"
+    exit 2
+fi
+
+cp -a inst-context ${OUT_DIR}
+
+echo -e "\n"
+
+echo "#####################build wasm apps"
+
+cd ${WASM_APPS}
+
+for i in `ls *.c`
+do
+APP_SRC="$i"
+OUT_FILE=${i%.*}.wasm
+
+# use WAMR SDK to build out the .wasm binary
+# require wasi-sdk with wasi-threads support. (wasi-sdk-20.0 or later)
+/opt/wasi-sdk/bin/clang     \
+        --target=wasm32-wasi-threads \
+        -pthread \
+        -Wl,--import-memory \
+        -Wl,--export-memory \
+        -Wl,--max-memory=655360 \
+        -o ${OUT_DIR}/wasm-apps/${OUT_FILE} ${APP_SRC}
+
+
+if [ -f ${OUT_DIR}/wasm-apps/${OUT_FILE} ]; then
+        echo "build ${OUT_FILE} success"
+else
+        echo "build ${OUT_FILE} fail"
+fi
+done
+echo "####################build wasm apps done"