Backport of scheduler: create placements for non-register MRD into release/1.4.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #15325 to be assessed for backporting due to the inclusion of the label backport/1.4.x.

The below text is copied from the body of the original PR.

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For multiregion jobs, the scheduler does not create placements on registration because the deployment must wait for the other regions. Once of these regions will then trigger the deployment to run.

Currently, this is done in the scheduler by considering any eval for a multiregion job as "paused" since it's expected that another region will eventually unpause it.

This becomes a problem where evals not triggered by a job registration happen, such as on a node update. These types of regional changes do not have other regions waiting to progress the deployment, and so they were never resulting in placements.

The fix is to create a deployment at job registration time. This additional piece of state allows the scheduler to differentiate between a multiregion change, where there are other regions engaged in the deployment so no placements are required, from a regional change, where the scheduler does need to create placements.

This deployment starts in the new "initializing" status to signal to the scheduler that it needs to compute the initial deployment state. The multiregion deployment will wait until this deployment state is persisted and its starts is set to "pending". Without this state transition it's possible to hit a race condition where the plan applier and the deployment watcher may step of each other and overwrite their changes.

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