Add information about autopilot health to the `/operator/autopilot/health` API
in Nomad Enterprise.
I've pulled the CE changes required for this feature out of @lindleywhite's PR
in the Enterprise repo. A separate PR will include a new `operator autopilot
health` command that can present this information at the command line.
Ref: https://github.com/hashicorp/nomad-enterprise/pull/1394
Co-authored-by: Lindley <lindley@hashicorp.com>
