(Optimization) remove popObjective, add resetObjectives, rename pushObjective =>...

(Optimization) remove popObjective, add resetObjectives, rename pushObjective => addObjective (#6634)

In order for OptimizationSolver to support pushing & popping,
we could remove popObjective because it might be difficult to handle cases like:

optSlv->pushObjective(...);
optSlv->push();
optSlv->popObjective();
optSlv->pop();
In this case we need to add back the popped objective...
If push/pop is supported, pop does not bring back objectives if you resetObjective
but it will revert the objs you add.
just like assertFormula and resetAssertions.