Nixops Provider Plugin Architecture

[johnalotoski]

This is to facilitate discussion on the topic of moving code for nixops providers out of the nixops repo and to their own repo by using a provider plugin architecture. @grahamc has done work on this already as seen in PR #981 and cites some reasons, such as (paraphrasing):

• A significant percentage of PRs are for providers and PRs to NixOps are slow to merge because:
  • It is super important that the providers in the core are stable and work well because users depend on it
  • For a lot of the providers, people don't really know how they work or interact with other stuff
  • It is hard to know how to add a new provider because everything is so tangled together
  • It is hard to add a new provider because it has to be in core

Migrating to a provider plugin architecture separate from the core Nixops repo may solve these issues and may also allow for further improvements or flexibility, such as other language support via plugin RPC.