Insight into the structure–function relationship of the nonheme iron halogenases involved in the biosynthesis of 4‐chlorothreonine – <scp>T</scp>hr3 from <i><scp>S</scp>treptomyces</i> sp. <scp>OH</scp>‐5093 and <scp>S</scp>yr<scp>B</scp>2 from <i><scp>P</scp>seudomonas syringae</i> pv. <i>syringae </i><scp>B</scp>301<scp>DR</scp>