Placeholder for the Fly.io Machines provider (planned for ExAtlas v0.2).
Fly.io Machines fits naturally alongside RunPod for this library's core use case: a Fly.io-hosted Phoenix app can spawn GPU Machines in the same region as itself for minimal-latency inference, using the ExAtlas API surface.
All callbacks currently return {:error, %ExAtlas.Error{kind: :unsupported}}.