Flowing data in System.CommandLine

There are a number within the System.CommandLine flow that could potentially supply information used by later steps. From one perspective this is a core part of the process because a return code (exit code) is returned that we anticipate will be the return value of Main.

#2156 discusses what should be returned from the flow. This the broader question about what state/data should be available during execution.

Several things could supply data that could be used by later steps:

* The calling code (often `static void Main(..)`
* Custom parsers
* Validators (beyond the validation failure info)
* Pre-actions (non-exclusive actions in #2147)
* CliAction (the action of the command, previously ICommandHandler)
* Post-actions if we have them (#2161)

In the old middleware pipeline, closures could be used to stash data between the before/after next step phases, but I do not think there were other ways to share state/data.

Have you ever wished a custom parser could stash some information for use by an action? Any other variation of wanting data to pass between parts of your parser?