added subscribe blocks example (#521)

* added subscribe blocks example

* print hash out to match parent hash

* Update examples/examples/rpc_call_subscribe_blocks.rs

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

* fmt + point to moved metadata

* tracing_subscriber now

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>
Co-authored-by: James Wilson <james@jsdw.me>

examples/examples/rpc_call_subscribe_blocks.rs

@@ -0,0 +1,65 @@
+// Copyright 2019-2022 Parity Technologies (UK) Ltd.
+// This file is part of subxt.
+//
+// subxt is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// subxt is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with subxt.  If not, see <http://www.gnu.org/licenses/>.
+
+//! To run this example, a local polkadot node should be running. Example verified against polkadot 0.9.18-f6d6ab005d-aarch64-macos.
+//!
+//! E.g.
+//! ```bash
+//! curl "https://github.com/paritytech/polkadot/releases/download/v0.9.13/polkadot" --output /usr/local/bin/polkadot --location
+//! polkadot --dev --tmp
+//! ```
+
+use subxt::{
+    rpc::Subscription,
+    sp_runtime::{
+        generic::Header,
+        traits::BlakeTwo256,
+    },
+    ClientBuilder,
+    DefaultConfig,
+    PolkadotExtrinsicParams,
+};
+
+#[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 api = ClientBuilder::new()
+        .set_url("wss://rpc.polkadot.io:443")
+        .build()
+        .await?
+        .to_runtime_api::<polkadot::RuntimeApi<DefaultConfig, PolkadotExtrinsicParams<DefaultConfig>>>();
+
+    // For non-finalised blocks use `.subscribe_blocks()`
+    let mut blocks: Subscription<Header<u32, BlakeTwo256>> =
+        api.client.rpc().subscribe_finalized_blocks().await?;
+
+    while let Some(Ok(block)) = blocks.next().await {
+        println!(
+            "block number: {} hash:{} parent:{} state root:{} extrinsics root:{}",
+            block.number,
+            block.hash(),
+            block.parent_hash,
+            block.state_root,
+            block.extrinsics_root
+        );
+    }
+
+    Ok(())
+}