Move get_ctx_singleton to a separate file

The get_ctx_singleton function returns the unique pointer of a GlobalRegistry.
To access the same GlobalRegistry in different languages, all functions has to access to the get_ctx_singleton function.

Currently, the only C and Fortran interface has access to the function.
This MR separates the function in a separate memman_ctx.hpp header so that C++ code can also access to the function without linking to the C interface library.