Implement list of Coder environments in new Toolbox architecture and include workspace actions

The newer Gateway architecture provides a way view environments in a list form (environments, in our case, being workspace agents).
In addition to having this list available to users, we also need a way to add workspace actions for each environment on the list, i.e. workspace _start_, _stop_, etc.

We should investigate if it is currently possible to decorate the list in Toolbox by reaching out to JetBrains. Otherwise, we should consider reimplementing this list from scratch so that we can add the actions ourselves.