cmd/atlas/internal/cmdapi: support reference env:// attributes

[a8m]

Added support for the env:// scheme, which allows for referencing attributes from the selected/current environment.

atlas schema diff --env drift --from env://state1 --to env://state2

atlas migrate diff --env dev --to env://state1

In the future, we can also add support for something like this flags block:

env {
  flags {
    schema {
      from = "..."
      to   = "..."
    }
  }
}