examples: Add chainHead_follow subscription example

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

examples/examples/chainhead_subscription.rs

@@ -0,0 +1,44 @@
+// Copyright 2019-2022 Parity Technologies (UK) Ltd.
+// This file is dual-licensed as Apache-2.0 or GPL-3.0.
+// see LICENSE for license details.
+
+//! To run this example, a local polkadot node should be running. Example verified against polkadot v0.9.28-9ffe6e9e3da.
+//!
+//! E.g.
+//! ```bash
+//! curl "https://github.com/paritytech/polkadot/releases/download/v0.9.28/polkadot" --output /usr/local/bin/polkadot --location
+//! polkadot --dev --tmp
+//! ```
+
+use sp_keyring::AccountKeyring;
+use subxt::{
+    tx::PairSigner,
+    OnlineClient,
+    PolkadotConfig,
+};
+
+#[subxt::subxt(runtime_metadata_path = "../artifacts/polkadot_metadata.scale")]
+pub mod polkadot {}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tracing_subscriber::fmt::init();
+
+    let signer = PairSigner::new(AccountKeyring::Alice.pair());
+    let dest = AccountKeyring::Bob.to_account_id().into();
+
+    // Create a client to use:
+    let api = OnlineClient::<PolkadotConfig>::new().await?;
+
+    // Subscribe to the `chainHead_follow` method.
+    let mut follow_sub = api.rpc().subscribe_chainhead_follow(false).await?;
+
+    // Handle all subscriptions from the `chainHead_follow`.
+    while let Some(event) = follow_sub.next().await {
+        let event = event?;
+
+        println!("event: {:?}", event);
+    }
+
+    Ok(())
+}