Changes to enable rust bindings

[ervanalb]

I forked this project so that I could write rust bindings for it (https://crates.io/crates/nemotron-asr). These are the changes that I made on my fork. I figured I might as well submit them upstream to see if you were interested in including them.

This PR is split up into 4 commits, each one being a fairly isolated change:

1. Make nemotron-asr.ccp agnostic to the GGML backend -- this change allows nemotron-asr.cpp to target any GGML backend, not just CPU, CUDA, and Metal. I personally tested Vulkan support which works great (substantially faster than CPU.) This also adds support for GGML_BACKEND_DL.
2. Add C interface for FFI -- I rewrote some of the C++ public interfaces in a C-style, allowing me to build a library libnemotron_asr.so / libnemotron_asr.a.
3. Add GGML as a git submodule -- this seems to be the standard for other projects using ggml (see for example, parakeet.cpp), and it simplifies the process of making a vendored build.
4. Enable static linkage and overriding variables in Makefile -- Rust programs are generally statically linked, so I wanted to allow for that option for libnemotron_asr and nemotron-asr.cpp. If ggml is built statically, it will be linked statically. Otherwise it will be linked dynamically.

Let me know if you'd like any changes to this PR--and it's also fine if you don't want to merge it at all. Thanks for this library, it's been working great!