Add void *context field to service data structures (#1265)

src/bacnet/rp.h

@@ -22,6 +22,7 @@ typedef struct BACnet_Read_Property_Data {
     int application_data_len;
     BACNET_ERROR_CLASS error_class;
     BACNET_ERROR_CODE error_code;
+    void *context; /* user-defined callback context */
 } BACNET_READ_PROPERTY_DATA;
 
 /* Forward declaration of RPM-style data structure */

src/bacnet/write_group.h

@@ -54,6 +54,7 @@ typedef struct BACnet_Write_Group_Data {
     BACNET_GROUP_CHANNEL_VALUE change_list;
     WRITE_GROUP_INHIBIT_DELAY inhibit_delay;
     struct BACnet_Write_Group_Data *next;
+    void *context; /* user-defined callback context */
 } BACNET_WRITE_GROUP_DATA;
 
 /**