Add ability to mark an agent inactive for new workspaces

MR: Pending

Description

As a administrator user who sets up Remote Development for my organization/personal-use, I want to be able to mark an agent inactive for new workspaces so that I can migrate to a new agent.

The agent would continue working normally for the existing workspaces. But it would not be allowed to create any new workspaces for the agent.

This would make it easier to transition/migrate between agents.