add support for ToolHandleArray to dual-use C++ configurables, rework handling subtools in AsgComponentConfig

This is part 1 of a request by @delsart to support configuring ToolHandleArray properties in dual-use code. Part 2 will be supporting it in the dual-use python configurables as well (in a separate MR).

For actually implementing this, I had to restructure AsgComponentConfig quite a bit. Originally I thought this would be easy enough to add to the existing logic, but it turns out it is a lot easier if each subtool is internally represented by an AsgToolConfig object instead.

cc @khoo @wbalunas @akraszna @sawyer @jroloff @jbossios