Currently the `PrepareExecutionHelper` abstract class uses a `std::unique_ptr<ResultType>` to return the planning results. Considering the recent changes, it is now overly complex and forces sub-classes to employ this awkward pattern for every method:

auto result = releaseResult();
// init result.
return result; 

We should strive to simplify this class as much as possible. Here's a non-exhaustive list of ideas:

• Get rid of std::unique_ptr, as there's no need for any indirection here.
• Consider constructing building the result directly in the targeted method. 
• Consider renaming this class to something more descriptive (ex: AbstractPlannerFactory?)
• Consider extracting `PrepareExecutionHelper...` classes to a separate file