Summary:
### Description
- Create custom readers and writers for index IO, which take function pointers as input
- Also expose these from the C_API
This is helpful for FFI use, where calling processes would pass upcall stubs for streamlined IO
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/4180
Reviewed By: gtwang01
Differential Revision: D71208266
Pulled By: mnorris11
fbshipit-source-id: ab82397d4780a2a07c7bfdc52329968377f42af4