Add `subsystem_name` kwarg to `ComponentSelector` interface

[GabrielKS]

get_components(::ComponentSelector, sys) currently cannot take a subsystem_name, I think because subsystems did not exist when I designed ComponentSelector. I have a PowerSimulations use case where it would be helpful to have this feature.

[GabrielKS]

Somewhat separately, it should be possible to make a ComponentSelector that encapsulates a subsystem name.