Implement dsl_sink_message_payload_debug_dir_get/set services to dump Message Converter payloads.

[rjhowell44]

New services for the IoT Message Sink

/**
 * @brief Enables payload dumping/debugging for the named Message Sink .
 * @param name unique name of the Message Sink to update.
 * @param[out] debug_dir absolute or relative path to the directory to
 * to dump payload data. 
 */
 DslReturnType dsl_sink_message_payload_debug_dir_get(const wchar_t* name, 
    const wchar_t** debug_dir);

/**
 * @brief Get the current payload dumping/debugging directory for the named 
 * Message Sink. Null string indicates that payload dumping is disabled. 
 * @param name unique name of the Message Sink to update.
 * @param[in] debug_dir absolute or relative path to the directory to
 * to dump payload data. 
 */
 DslReturnType dsl_sink_message_payload_debug_dir_set(const wchar_t* name, 
    const wchar_t* debug_dir);