refactor: jamfpro_computer_prestages.go - clean up ptr usage and add omitempty for optional fields per API spec

[neilmartin83]

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Reviewed the endpoint and refactored fields to remove excessive pointer usage. Should make it more robust and efficient. Have also ensured optional fields as specified in the API schema are omitted. https://developer.jamf.com/jamf-pro/reference/post_v3-computer-prestages

Added a new skipSetupItems field for OSShowcase.

No changes to methods - only updated types and example scripts as needed.

This will require the provider resource to be refactored to remove pointer handling for fields that no longer require it. Will submit a PR for this shortly.

The main advantage here is future proofing for new fields that appear in skipSetupItems - found that in testing, the API handles any field name supplied here safely. If you try to set one in a request body that doesn't exist in that version of JPro, it's ignored for POST/PUT. This would cause issues previously when using bool pointers.

The API is also tolerant when sending top-level fields introduced in new versions of Jamf Pro well - like PlatformSsoAppBundleId and PssoEnabled introduced in 11.21, the request succeeds and the field is ignored.

Also removed many non-existent fields from ResourceComputerPrestage and the OnboardingItem type which is also not in the API spec.

I have tested against Jamf Pro 11.20, 11.21 and 11.22 with no issues.

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