Add dependencies on axum and tokio and implement a do-nothing server

Add dependencies on axum and tokio and implement a do-nothing server

This server binds to port 3000 on all interfaces and then just listens,
(with no routes configured to do anything).

The axum library gives us the server and router, (which, like I said,
we aren't using yet). And tokio gives us some network functionality
(see the TCP listener and the bind function) as well as some
asynchronous functionality, (see the #[tokio::main] attribute which
saves bunch of boilerplate that would otherwise be needed (creating a
tokio::runtime::Builder and giving it a function to run), and instead
lets us just use the entire body of main as the async function body
that tokio launches.