Enable remote attestation by librats in SGX mode (#1445)

Add library librats, update SGX build scripts, add sample and update document.

core/iwasm/libraries/lib-rats/lib_rats.cmake

@@ -0,0 +1,33 @@
+# Copyright (c) 2022 Intel Corporation
+# Copyright (c) 2020-2021 Alibaba Cloud
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+set (LIB_RATS_DIR ${CMAKE_CURRENT_LIST_DIR})
+
+add_definitions (-DWASM_ENABLE_LIB_RATS=1)
+
+include_directories(${LIB_RATS_DIR})
+
+include(FetchContent)
+
+set(RATS_BUILD_MODE "sgx"
+    CACHE INTERNAL "Select build mode for librats(host|occlum|sgx｜wasm)")
+set(RATS_INSTALL_PATH  "${CMAKE_BINARY_DIR}/librats" CACHE INTERNAL "")
+
+FetchContent_Declare(
+    librats
+    GIT_REPOSITORY https://github.com/inclavare-containers/librats
+    GIT_TAG master
+)
+FetchContent_GetProperties(librats)
+if (NOT librats_POPULATED)
+    message("-- Fetching librats ..")
+    FetchContent_Populate(librats)
+    include_directories("${librats_SOURCE_DIR}/include")
+    add_subdirectory(${librats_SOURCE_DIR} ${librats_BINARY_DIR} EXCLUDE_FROM_ALL)
+
+endif()
+
+file (GLOB source_all ${LIB_RATS_DIR}/*.c)
+
+set (LIB_RATS_SOURCE ${source_all})