Zephyr: Support C++ builds

Some of the Zephyr include files need to be C++, not C based. The 'extern "C"'
wrapper in include/coap3/coap.h means that all #include are treated as being C.

Remove 'extern "C"' wrapper from include/coap3/coap.h and add wrapper to
include files after #include and before any function / structure definitions.

Add in a C++ build test for CI.

examples/zephyr/test-cpp-src/Kconfig

@@ -0,0 +1,8 @@
+# Private config options for coap-client sample app
+
+# Copyright (c) 2020 Intel Corporation
+# SPDX-License-Identifier: Apache-2.0
+
+mainmenu "Test cpp build application"
+
+source "Kconfig.zephyr"