Refactor objective in optimizer

There is a need to evaluate graphs and pipelines in side mutations from optimizer (e.g. Sructural Analysis). On the one hand, current `objective` property does not allow to do such evaluations since there is no referenced data in it. And on the other hand, `self.eval_dispatcher._objective_eval` is fine for evaluating pipelines, however this function does not provide any information about calculated metric's names.
That is precisely why `objective` should be modified. Current solutions is to construct new mixed objective like this:
`objective` is this class is `self.eval_dispatcher._objective_eval`