Implement push for devcontainer state changes

[mafredri]

Currently devcontainer state changes are detected in the dashboard via 10s polling. This creates an artificial delay for the state to be reflected in the UI.

We should implement a push-signal to notify clients of devcontainer state changes (dirty status, recreated, etc). This could be done via WebSockets or triggering a workspace update via pubsub indicating resources should be refreshed.

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We may also want to propagate the signal to editor integrations, but we should be vary of duplicating logic (e.g. VS Code watches the devcontainer.json for changes already, adding our own notifications on top may be excessive).