feat: add a broadcast channel for job state event notifications

[Jeadie]

Which issue does this PR close?

None Closes #. Suggested by @milenkovicm that we should upstream our changes (see comment).

Rationale for this change

Useful in spiceai/datafusion-ballista for production features.

What changes are included in this PR?

Summary

Adds a tokio::sync::broadcast sender to SchedulerServer, subscribable via subscribe_job_updates(). On job state changes (queued / running / completed / failed / cancelled) the scheduler broadcasts a JobStateEvent carrying the job id and new state. Lets components holding a handle to the scheduler observe job lifecycle transitions without polling.

Relationship to the existing JobStatusSubscriber

Similar to the existing JobStatusSubscriber (mpsc) mechanism wired through submit_job(..., subscriber). This broadcast channel is a separate, fan-out style notification path; worth deciding the best DX between these.

	JobStatusSubscriber (existing)	JobStateEvent broadcast (this PR)
Channel	mpsc - single consumer	broadcast - fan-out
Scope	1 subscriber per submitted job, request-scoped	Global; fires for all jobs
Consumer	submitting gRPC client, via ExecuteQuery / GetJobStatusResult.	in-process component(s), via subscribe_job_updates()
Payload	Full protobuf JobStatus	Lightweight JobStateEvent { job_id: String, state }

Both fire from the same lifecycle points in query_stage_scheduler.rs.

Ported from spiceai#15.