Support triggers between workspaces

[matthewbarr]

Feature description 💡

It would be great if it was possible to trigger a plan / plan&apply / plan & approve & apply etc in another workspace.

We would be able to get rid of our CI system for the ordered deployment of a change between the dev account (workspace), staging, and prod.

( it might be possible to do this now, but using VCS for the dev, and then api calls for staging and prod.)

Anything else?

In the future, we're expecting stacks from hashiconf, to vaguely group multiple workspaces of the same module, for different envs or regions. This can be a starting block for connecting workspaces.