When generating .pxd, warn for public but unexported functions

It's possible to add a function to `inline_array.h` or other header included by `nanoarrow.h` which is nevertheless not included in generated `.pxd`. The error is then simply "no matching function" when compiling the `.pxd`, which is not informative. It'd be better if the pxd could recognize function definitions or declarations which aren't translatable into a `cdef`, then warn if those aren't on an exclusion list.

Start by locating the generator that produces the .pxd from nanoarrow.h and inspect how inline_array.h and other included headers are translated. Trace how exclusions are represented, then determine how warnings should identify public declarations that cannot become cdef entries and verify the behavior with the generator's existing checks or tests.

Written by the indexing model from the issue text.