Figure out plugin protocol versioning evolution

[f0rmiga]

Right now, we don't test or support older protocol versions. We need to come up with a better story for this. Perhaps the SDK could do some work when a plugin is too old and doesn't support a specific method? This task is broad and the outcome should be a small design doc with some POC.

[alexeagle]

Note that terraform has an extensive doc on versioning: https://github.com/hashicorp/terraform/tree/main/docs/plugin-protocol

I'd strongly prefer that we follow that as much as possible - part of our decision to use the hashicorp go-plugin system was that this is a solved problem.