Add additional reverse-proxy example, using RFC 8075 style template URI support

The current reverse-proxy example differs in quite some aspects from the (maybe more usual?) type of reverse proxy that is defined by RFC 8075 for HTTP-to-CoAP. We could add an example of a reverse-proxy that operates with such a template URI. The proxy in this case uses only one IP address i.e. endpoint. From this endpoint it can serve both group requests and proxied unicast requests.