Relates to: coder/coder#10352
Blocked by: #1093

When a coder script (or other resource) updates its status (eg. from running to completed), other resources that depend on it may become eligible to run. These dependencies are tracked in https://github.com/coder/coder/issues/20407.

The scope of the current issue is to add a pub/sub mechanism to the graph that was implemented in https://github.com/coder/coder/issues/20407, so that subscribers can be notified when they have been unblocked. Existing pub/sub mechanisms should be reused where possible and appropriate. Subscribing and publishing should be in-process only for this issue. Exposure to specific APIs (CLI, terraform) will follow in subsequent issues.