Allow the date/time in the security headers to be spoofed

When using ONVIF with ws-security to talk to digital cameras, the cameras are not always time synced with the system. After polling the camera's current date/time I need to spoof my the times in the security headers in order to properly set the camera's date/time. Once that is done then I can use the system's clock for the security timestamps. This change adds a time source that can be overridden to return a fake system time that is consistent with the current time in the camera