Feature request: allow load/unload models on server

[ngxson]

I extract this discussion from #13367 , mainly for better planning tasks around this.

allow loading / unloading model via API: in server.cpp, we can add a kinda "super" main() function that wraps around the current main(). The new main will spawn an "interim" HTTP server that expose the API to load a model. Ofc this functionality will be restricted to local deployment to avoid any security issues.

This idea has been demo in #13400 , but the implementation is still far from usable. It actually requires a refactoring of server.

While alternative methods for hot-swapping model already exist, I think refactoring the server.cpp code can still benefit the long-term development quite a lot. Therefore, this feature can potentially be a suitable goal for the refactoring efforts.