gh-141388: Improve support for non-function callables as annotate functions

[dr-carlos]

• Clarified in the glossary that an annotate function can be any callable (as is already the case).
• Improved error messages if call_annotate_function() tries to run with fake globals but can't find the annotate function's __code__ attribute.
• Added defaults if other non-required attributes cannot be found for fake globals, e.g. if the annotate function doesn't provide __defaults__, that's fine, just assume it's None.
  If required, I can remove some (or all) of these defaults, particularly __globals__ which adds an extra parameter to _build_closure(), or __builtins__ which is semantically correct but a little more complicated than the rest (it would be much simpler to just fallback to builtins.__dict__).
• Added tests for the current support, added support, and error messages associated with non-function callables as annotate functions..
• Added a section to the annotationlib documentation providing a simple recipe for a 'minimum viable implementation' of a custom callable class as annotate function. Happy to remove if this is unnecessary/too niche - I just thought it was the clearest way of demonstrating how to implement one of these, without just spelling out which dunders they need to implement.

• Issue: __annotate__ Functions don't actually need to be functions #141388

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📚 Documentation preview 📚: https://cpython-previews--142327.org.readthedocs.build/

[sobolevn]

I am not quite convinced that this should be done. __annotate__ is already complex enough.

In the issue itself there were no real use-cases that can support the idea.

In my opinion, it is fine to have __annotate__ magic function as function only.