Support Static File Dir/Root Dir in Server

**Is your feature request related to a problem? Please describe.**
The `server` in `llama_cpp_python` did not embed a UI for chat, which would be convenient in debugging/testing.

**Describe the solution you'd like**
While a lightweight web UI could be easily composed in a few hundred lines of vanilla HTML/js, modern browsers usually impose cross origin limitations for many APIs, and force the requests to be proxy-ed to align their origins with the web page, which increased the workload to set up the minimal environment.

However, the FastAPI which was imported in `server` module already provided support for `StaticFiles` which is perfect to host a lightweight web UI and other stuff. To achieve this, server settings should be augmented to include an argument for the path of static files or root, and the FastAPI `app` should `mount` it in `create_app()`.

**Describe alternatives you've considered**
N/A

**Additional context**
N/A