Consider changing config code to use figment

[stevenengler]

Shadow's configuration code currently uses clap to read cli options and serde_yaml to read yaml files. Then we do some merging to combine the two. But this makes the configuration code a bit awkward.

The figment library claims to handle this, so it might be nice to try it to simplify our configuration code. It's not immediately clear if it would support everything required by Shadow (for example including the default values in the clap help text like we do with generate_help_strs).

[stevenengler]

Closing since the current version works fine, it's not obvious if it will be an improvement, and I doubt anyone would volunteer to work on this issue, so it's probably best to have one fewer forever-issue.