[MXNet Extensions] Include `lib_api.h` in the pre-built pip package

[wkcn]

Description

Thanks for the effort on the support of custom operators (PR #15760, #15921, #15886, #17034, #17194, #17241, #17486. #17516, #17569, #17585, #17885 etc.), which makes it easy to implement custom operators in seperate libraries and load them at runtime.

It need the header file lib_api.h to build the libraries. Some users use the pre-built MXNet (e.g. pip package) which doesn't contain the header file. If they use the specific version of MXNet, they need to find the matched version of lib_api.h.

It will be more convenient if the pre-built MXNet contains the header file lib_api.h.

cc: @samskalicky @rondogency
Have you got any idea? Thank you!

[rondogency]

@wkcn Thanks for bringing this up. It would be a great improvement for custom operator usability. We can discuss more later to see if this feature could go into 2.0 release