Seems HostMonitor option in host.json not working at all

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By setting `"healthMonitor": { "enabled": false }` we still got Host shutdown due to unhealthy.
By static code review, it seems you doesn't consume `host.json` in WebJobs host, but only consume they in ScriptHost:
https://github.com/Azure/azure-functions-host/blob/0120ead6bbf821ffa8bac7f9ddd8b57458ccce43/src/WebJobs.Script/ScriptHostBuilderExtensions.cs#L104
This `HostJsonFileConfigurationSource` was only added in `DefaultScriptHostBuilder.BuildHost` but not added in WebJobsScript, and therefore the healthMonitorOptions could not get configuration from host.json at all.
https://github.com/Azure/azure-functions-host/blob/0120ead6bbf821ffa8bac7f9ddd8b57458ccce43/src/WebJobs.Script.WebHost/WebJobsScriptHostService.cs#L82

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Start by reading the configuration setup in src/WebJobs.Script/ScriptHostBuilderExtensions.cs around the linked line, then compare it with src/WebJobs.Script.WebHost/WebJobsScriptHostService.cs. Trace how host.json reaches healthMonitorOptions and verify that setting healthMonitor.enabled to false prevents shutdown due to an unhealthy host; add or run the relevant regression coverage if present.

Written by the indexing model from the issue text.