What should we add?

Right now the HighLevelSynthesisPlugin interface only allows for a plugin to return a QuantumCircuit, however this needlessly forces a conversion when HighLevelSynthesis is run as when the pass is being used we're always in a dag context. We can't change the interface, but we should expand it to support both a dag and circuit return from a plugin and just normalize to a dagcircuit inside the pass. So for passes that use a dagcircuit we avoid the conversion.