ICU-23447 MessageFormat ICU4C: Make Function* pointers const

[catamorphism]

Change MFFunctionRegistry::getFunction() to return const Function* instead of Function*, with Function::call() becoming const.

Checklist

• Required: Issue filed: ICU-23447
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• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

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[srl295]

Need approval of my additional commits:

• merged from main- fixed a templating warning
• brought up to sync with ICU-23424 C: MF2: Allow overriding standard functions #4041

I can't squash because i'm not @catamorphism but would like to merge this so it can be the basis of the updated MF2 C++ API proposal.