Add capacity for linking model in the same way as goto-cc for C++/Rust API

[NlightNFotis]

Description

Add support for linking models from separate files (and potentially writing the resulting model into a separate file on disk), and specialising with the --function.

Ideally, the end result should allow interaction with the API that achieves the same outcome that the following two calls achieve:

1. goto-cc a.out /home/ubuntu/git/kani/library/kani/kani_lib.c -o b.out
2. goto-cc b.out --function harness -o b_harness.out

Context

[This is to track items outlined in #7042 that are missing to make the C++ and Rust API more useful]