add operator API to reject all job registrations

[tgross]

During incident response, operators may find that automated processes elsewhere in the organization can be generating new workloads on Nomad clusters that are unable to handle the workload. Add an operator API endpoint that causes all job registration calls to be rejected with an error stating that the cluster is in a load-shedding mode.

[tgross]

We've also had an internal discussion about automatically load-shedding in cases where the Nomad servers are overloaded, but that's a good deal more involved than this issue. In any case, whatever state this option has will most likely be used for that mechanism anyways.

[tgross]

My thinking on the approach for this:

• A new field RejectJobRegistration on SchedulerConfig, which we already have wired up to get persisted to raft.
• Job.Register and Job.Dispatch will check this this field value. If the field is true we'll return an error unless the ACL is a management token.
• We don't currently allow SchedulerConfig to be set via CLI... we may have a couple more upcoming features for SchedulerConfig, so let's put off adding a CLI until we know what they're all going to be so that we have a sensible/ergonomic CLI for all of them together.

[github-actions]

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