Simplify tls-rustls example (#254)

## Motivation

Current `tls-rustls` example might be inconvenient for some people.

## Solution

Rename current example to `low-level-rustls` and add a high level example in its place.

examples/low-level-rustls/src/main.rs

@@ -0,0 +1,68 @@
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-low-level-rustls
+//! ```
+
+use axum::{handler::get, Router};
+use hyper::server::conn::Http;
+use std::{fs::File, io::BufReader, sync::Arc};
+use tokio::net::TcpListener;
+use tokio_rustls::{
+    rustls::{
+        internal::pemfile::certs, internal::pemfile::pkcs8_private_keys, NoClientAuth, ServerConfig,
+    },
+    TlsAcceptor,
+};
+
+#[tokio::main]
+async fn main() {
+    // Set the RUST_LOG, if it hasn't been explicitly defined
+    if std::env::var("RUST_LOG").is_err() {
+        std::env::set_var("RUST_LOG", "example_tls_rustls=debug")
+    }
+    tracing_subscriber::fmt::init();
+
+    let rustls_config = rustls_server_config(
+        "examples/tls-rustls/self_signed_certs/key.pem",
+        "examples/tls-rustls/self_signed_certs/cert.pem",
+    );
+
+    let acceptor = TlsAcceptor::from(rustls_config);
+    let listener = TcpListener::bind("127.0.0.1:3000").await.unwrap();
+
+    let app = Router::new().route("/", get(handler));
+
+    loop {
+        let (stream, _addr) = listener.accept().await.unwrap();
+        let acceptor = acceptor.clone();
+
+        let app = app.clone();
+
+        tokio::spawn(async move {
+            if let Ok(stream) = acceptor.accept(stream).await {
+                let _ = Http::new().serve_connection(stream, app).await;
+            }
+        });
+    }
+}
+
+async fn handler() -> &'static str {
+    "Hello, World!"
+}
+
+fn rustls_server_config(key: &str, cert: &str) -> Arc<ServerConfig> {
+    let mut config = ServerConfig::new(NoClientAuth::new());
+
+    let mut key_reader = BufReader::new(File::open(key).unwrap());
+    let mut cert_reader = BufReader::new(File::open(cert).unwrap());
+
+    let key = pkcs8_private_keys(&mut key_reader).unwrap().remove(0);
+    let certs = certs(&mut cert_reader).unwrap();
+
+    config.set_single_cert(certs, key).unwrap();
+
+    config.set_protocols(&[b"h2".to_vec(), b"http/1.1".to_vec()]);
+
+    Arc::new(config)
+}