Use Hessian approximation in CyIpopt when grey-box without Hessian is present

[Robbybp]

Fixes #3891

Changes proposed in this PR:

• Set (potentially override) hessian_approximation=limited-memory in CyIpopt solver when the model contains a grey-box that doesn't support Hessians
• Log a warning if this happens and the user had set hessian_approximation=exact
• Add a has_hessian_support method to PyomoNLPWithGreyBoxBlocks (previously only existed on _ExternalGreyBoxAsNLP, although the former has a private _has_hessian_support attribute)

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