Changes to Add legal hold custodians

changed the pattern of the path request parameter hold_id from ^lgh_[a-zA-Z0-9]{14}$ to ^[Ll][Gg][Hh]_[a-zA-Z0-9]{14}$

changed the pattern of the request property custodians/items/id from ^usr_[a-zA-Z0-9]{14}$ to ^[Uu][Ss][Rr]_[a-zA-Z0-9]{14}$

endpoint added