Add a benchpark variant list command

[dyokelson]

Now that the experiments are structured to inherit types of experiments (cuda, rocm, caliper, scaling, etc) there are a lot of variants to the experiments that aren't exposed just in the experiment.py, but instead "hidden" in these parent classes.

This means that to find all the variants for an experiment you have to see what it inherits, find the parent class in the code and then look at the variant options.
Some sort of command such as benchpark variants saxpy that lists all possible variants and their optional values, whether explicitly defined or inherited, for a specific experiment class would be useful for users to easily identify any possible parameters they need/can pass in.