ProgressHandler support in C API

[krishty]

Assimp exports the ProgressHandler class and offers Importer::SetProgressHandler() to set a custom progress handler.

This, however, requires a C++ class with virtual functions and there is no way (to my knowledge) to use this functionality from the C interface in cimport.h.

I.e. from C, it is impossible to

1. display progress information
2. cancel imports.

It would be handy to have C support for progress: Let Assimp internally derive ProgressHandler to store a pair of (C function pointer + user context) similar to struct aiFileIO, then forward all updates to the C function pointer. I don’t know how that pair would be passed to aiImportFileFromMemoryWithProperties() without breaking the API, though.

[tellypresence]

Is this related to #5190?

[kimkulling]

Yes, it is.

[tellypresence]

Feature request issues tracked via epic #6183