LwIP: Add in TinyDTLS support

The dtls versions of the server and client are built as
server-dtls and client-dtls.

Add in flexible option support for server and client.  These are
a subset of the coap-server and coap-client options.

.github/workflows/main.yml

@@ -127,7 +127,7 @@ jobs:
           $GITHUB_WORKSPACE/configure --disable-documentation --disable-examples --disable-tests --disable-dtls
       - name: compile
         run: |
-          make -C examples/${{matrix.OS}}
+          make -C examples/${{matrix.OS}} EXTRA_CFLAGS=-Werror
   ms-build:
     runs-on: windows-latest