Improve Method for Shutting Down Shortfin LLM Server

[stbaione]

Description

Currently, we don't have a good way to shutdown the LLM server in shortfin. The only routes that exist are either:

CTRL + C

If process is running in current terminal, OR

kill -9 <pid>

If process is running in the background or in another terminal.

We should have an easier way to shutdown a running server. A good suggestion for this from @ScottTodd is to add a cli command, similar to the way that torchserve cli does it:

torchserve --stop