Add optional conversion complete callback to `AdcHandle::Start()`

[TheSlowGrowth]

This PR adds a "conversion complete" callback to the AdcHandle similar to the ones found in other drivers. This would allow users to actually know when a conversion is complete, as opposed to blindly reading the data off the AdcHandle without knowing if it's actually new. A different solution would be to provide a "running counter" that increments on each completed conversion, as a means of identifying from the outside that new data is available. This PR is just a proposal, of course. Happy to discuss other approaches!