Currently exec::agg::buildPipeline returns exec::agg::Pipeline by the means of std::unique_ptr.

There is no specific reasons on the 'returning' side to return std::unique_ptr, so we can switch to returning by value, as generally recommended.

So far, the default copy constructor is explicitly deleted, and this is desired to avoid accidental copying. The exec::agg::buildPipeline only needs to be moved, unless we have some new use-cases.