Study protocol for a multicentre randomised controlled trial:<i>S</i>afety,<i>T</i>olerability, efficacy and quality of life<i>O</i>f a human recombinant alkaline<i>P</i>hosphatase in patients with sepsis-associated<i>A</i>cute<i>K</i>idney<i>I</i>njury (STOP-AKI)