examples: configure KeyLogFile for all examples.

The example programs distributed with Rustls are often used to
demonstrate TLS features, or debug issues, that require being able to
view plaintext data captured in a pcap (e.g. with Wireshark).

To make this convenient this commit updates the examples (minus the MIO
examples that already did so ) to configure Rustls with a `KeyLogFile`
implementation of the `KeyLog` trait. Users can then specify the
`SSLKEYLOGFILE` environment variable to log the required session secrets
for use with their pcap tool of choice.

examples/src/bin/server_acceptor.rs

@@ -228,16 +228,19 @@ impl TestPki {
         // Build a server config using the fresh verifier. If necessary, this could be customized
         // based on the ClientHello (e.g. selecting a different certificate, or customizing
         // supported algorithms/protocol versions).
-        Arc::new(
-            ServerConfig::builder()
-                .with_safe_defaults()
-                .with_client_cert_verifier(verifier)
-                .with_single_cert(
-                    vec![Certificate(self.server_cert_der.clone())],
-                    PrivateKey(self.server_key_der.clone()),
-                )
-                .unwrap(),
-        )
+        let mut server_config = ServerConfig::builder()
+            .with_safe_defaults()
+            .with_client_cert_verifier(verifier)
+            .with_single_cert(
+                vec![Certificate(self.server_cert_der.clone())],
+                PrivateKey(self.server_key_der.clone()),
+            )
+            .unwrap();
+
+        // Allow using SSLKEYLOGFILE.
+        server_config.key_log = Arc::new(rustls::KeyLogFile::new());
+
+        Arc::new(server_config)
     }
 
     /// Issue a certificate revocation list (CRL) for the revoked `serials` provided (may be empty).

examples/src/bin/simple_0rtt_client.rs

@@ -77,6 +77,9 @@ fn main() {
         .with_root_certificates(root_store)
         .with_no_client_auth();
 
+    // Allow using SSLKEYLOGFILE.
+    config.key_log = Arc::new(rustls::KeyLogFile::new());
+
     // Enable early data.
     config.enable_early_data = true;
     let config = Arc::new(config);

examples/src/bin/simpleclient.rs

@@ -28,11 +28,14 @@ fn main() {
                 )
             }),
     );
-    let config = rustls::ClientConfig::<Ring>::builder()
+    let mut config = rustls::ClientConfig::<Ring>::builder()
         .with_safe_defaults()
         .with_root_certificates(root_store)
         .with_no_client_auth();
 
+    // Allow using SSLKEYLOGFILE.
+    config.key_log = Arc::new(rustls::KeyLogFile::new());
+
     let server_name = "www.rust-lang.org".try_into().unwrap();
     let mut conn = rustls::ClientConnection::new(Arc::new(config), server_name).unwrap();
     let mut sock = TcpStream::connect("www.rust-lang.org:443").unwrap();