A client can send a `SocketIOSubscription` object with the
`/subscription` event type, in order to manage the subscriptions to
job-related updates.
Subscribing to a job means receiving updates to its tasks. General job
updates are sent to all clients anyway.
This commit only contains the OpenAPI YAML change, to see if it works
well to separate such commits into three parts:
- OpenAPI definition change.
- Regenerated code changes.
- Hand-written code changes.
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