Handle Scaling Policies in Job Plan endpoint

[notnoop]

Fixes #8544

This PR fixes a bug where using nomad job plan ... always report no change if the submitted job contain scaling.

The issue has three contributing factors:

1. The plan endpoint doesn't populate the required scaling policy ID; unlike the job register endpoint
2. The plan endpoint suppresses errors on job insertion - the job insertion fails here, because the scaling policy is missing the required ID
3. The scheduler reports no update necessary when the relevant job isn't in store (because the insertion failed)

This PR fixes the first two factors. Changing the scheduler to be more strict might make sense, but may violate some idempotency invariant or make the scheduler more brittle.

I added the test early, so reviewer can see the failing TestJobEndpoint_Plan_Scaling test in https://app.circleci.com/pipelines/github/hashicorp/nomad/10845/workflows/f535e389-633b-46db-b304-353c79c3894e/jobs/87394 but is green afterwards.

[jrasell]

LGTM thanks for jumping on this @notnoop! Tested locally and can confirm it fixes the linked issue.

[github-actions]

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