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.

.github/workflows/main.yml

@@ -165,7 +165,7 @@ jobs:
       - uses: actions/checkout@v4
       - name: setup
         run: |
-          sudo apt-get update && sudo apt-get install -y libc6-dev-i386
+          sudo apt-get update && sudo apt-get install -y libc6-dev-i386 lib32stdc++-13-dev
       - name: compile
         run: |
           make -C examples/${{matrix.OS}} EXTRA_CFLAGS=-Werror