Separate Darc and PCS codeflow logic from VmrForwardFlower/VmrBackflower

[dkurepa]

Context

There are two ways to use the codeflow: through CLI and through PCS. For forward flow, we have a class called VmrForwardFlower that implements most of the flow logic. In the CLI scenario, we directly call into this class and preform what we need to.
In the PCS scenario, we have a class on top of this one PcsVmrForwardFlower that does some things of it's own, and then calls into the VmrForwardFlower. This inconsistency makes it a bit hard to read the code.
The same goes for the backwards flow

Goal

Make VmrForwardFlower and VmrBackwardFlower abstract, and add DarcForwardFlower/DarcBackwardFlower to be a class similar to the currently existing PcsVmrFlower classes