Passing context into parsed commands

I'd like to add some context into every command. Ideally, initialize it with objects.

Currently [`ParsableArguments`](https://github.com/apple/swift-argument-parser/blob/main/Sources/ArgumentParser/Parsable%20Types/ParsableArguments.swift#L30) requires `init()`. SAP uses it to instantiate commands once input has been parsed.

Is there a way to inject e.g. DI into parsed command? I can always create a singleton and then get it from command instance, but I'd love to `init(with di: MyDI)` instead. Also, I can parse `: ParsableArguments`, but I'd love to keep command tree feature available, and that works only with `ParsableCommand` and configurations as far as I can tell.

Start with Sources/ArgumentParser/Parsable Types/ParsableArguments.swift, especially the init() requirement, and trace how SAP instantiates commands after parsing. Compare ParsableArguments parsing with ParsableCommand configurations and define an injection approach that preserves the command-tree feature; the issue is done when the supported behavior and initialization contract are established.

Written by the indexing model from the issue text.