We have the public API `JsonObject.ConvertFromJson` to convert from JSON string in the PowerShell context. It would be good to have a public API for conversion to JSON. This PR refactors the `ConvertTo-Json` cmdlet to move the core implementation to `JsonObject.ConvertToJson`, and make `ConvertTo-Json` call that public method.
This would help the Azure Function PowerShell worker. Currently, we depends on [calling the cmdlet](729710d259/src/PowerShell/PowerShellManager.cs (L198-L205)) to convert object to JSON which is expensive. Once we have the public method `JsonObject.ConvertToJson` exposed, we can call the API directly to avoid a command invocation.
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