webrtc: add an answerer example for the web client's e2e

Reads a `webrtc://` offer from stdin, answers it, bridges ICE candidates
as JSON lines over stdin/stdout and echoes every message. rustdesk-web-js
drives it from Chrome (js/e2e/webrtc) to check that the browser transport
and WebRTCStream still agree on the envelope, the signed fingerprint, the
candidate JSON and the fragment framing.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JXJJGEGdgu26wgCppvUXdZ

examples/webrtc_echo.rs

@@ -0,0 +1,80 @@
+// Temporary e2e helper for the web client: answers a browser offer read from
+// stdin, bridges ICE candidates over stdin/stdout as JSON lines, then echoes
+// every message it receives.
+extern crate hbb_common;
+
+#[cfg(feature = "webrtc")]
+#[hbb_common::tokio::main]
+async fn main() -> hbb_common::anyhow::Result<()> {
+    use hbb_common::{
+        anyhow::anyhow,
+        serde_json::{self, json, Value},
+        tokio,
+        webrtc::WebRTCStream,
+    };
+    use std::io::BufRead;
+
+    let mut offer = String::new();
+    std::io::stdin().lock().read_line(&mut offer)?;
+    if offer.trim().is_empty() {
+        return Err(anyhow!("no offer"));
+    }
+    let mut stream = WebRTCStream::new(offer.trim(), false, 30000).await?;
+    let out = json!({
+        "answer": stream.local_endpoint(),
+        "session_key": stream.session_key(),
+        "local_fingerprint": stream.local_dtls_fingerprint().await?,
+        "remote_fingerprint": stream.remote_dtls_fingerprint().await?,
+    });
+    println!("{}", out);
+    let mut ice_rx = stream
+        .take_local_ice_rx()
+        .ok_or_else(|| anyhow!("no ice rx"))?;
+    tokio::spawn(async move {
+        while let Some(c) = ice_rx.recv().await {
+            println!("{}", json!({ "candidate": c }));
+        }
+    });
+    let (tx, mut rx) = tokio::sync::mpsc::unbounded_channel::<String>();
+    std::thread::spawn(move || {
+        let stdin = std::io::stdin();
+        for line in stdin.lock().lines().flatten() {
+            if tx.send(line).is_err() {
+                break;
+            }
+        }
+    });
+    let for_ice = stream.clone();
+    tokio::spawn(async move {
+        while let Some(line) = rx.recv().await {
+            let Ok(v) = serde_json::from_str::<Value>(&line) else {
+                continue;
+            };
+            if let Some(c) = v.get("candidate").and_then(Value::as_str) {
+                if let Err(e) = for_ice.add_remote_ice_candidate(c).await {
+                    eprintln!("add_remote_ice_candidate failed: {e}");
+                }
+            }
+        }
+    });
+    stream.wait_connected(30000).await?;
+    println!(
+        "{}",
+        json!({
+            "connected": true,
+            "relayed": stream.is_relayed().await,
+            "remote_ipv6": stream.is_remote_ipv6().await,
+        })
+    );
+    while let Some(res) = stream.next().await {
+        let bytes = res?;
+        let len = bytes.len();
+        stream.send_bytes(bytes.freeze()).await?;
+        eprintln!("echoed {len} bytes");
+    }
+    stream.close().await;
+    Ok(())
+}
+
+#[cfg(not(feature = "webrtc"))]
+fn main() {}