code: @POST
    @Path("{id}/username")
    @RolesAllowed({"ROLE_ADMIN"})
    public Response changeUsername(@PathParam("id") Long id, UsernameRequest usernameRequest) {
      checkUsernameFormat(usernameRequest.username);
  
      userService.changeUsername(id, usernameRequest.getUsername());
  
      return Response.ok(id).build();
    }
docs: Change a users username.
  The username must be unique
  
  @param usernameRequest new username
  @return 200 if success
  409 if username is not unique