[Proposal] Extending Deployment to allow more flexible environments

[gian]

The predefined wisp.deployment environments---i.e., those implied by Deployment#mapToEnvironmentName()---are pretty stubborn in their belief that I have exactly one environment called staging. This flows over into the behavior of MiskConfig in resolving config file names and such. My specific use case would benefit from being able to have more flexibility here - e.g., being able to define deployments that map to something like staging1 and staging2, or perhaps a totally different environment not contemplated in the fixed set.

To be clear, I'm not proposing meddling with properties like isStaging or isTest - the actual categories of deployments are totally fine. I'd just like more flexibility in how those deployment types map to named environments for purposes like config or cluster granularity.

I'd be happy to attempt a small enhancement PR here, but I first wanted to check if a) there was any appetite for this whatsoever, and b) if there was some other preferred way to accomplish this.

(aside: Thanks for the continued open-source development of Misk! It's nice being able to still benefit from so many of the things I liked about writing software at Block even after leaving)