Add legacy RPC usage example (#1279)

* Add example using legacy rpc

* Illustrate how account_nonce and system_acc_next eventaully cathces up

* Adapt and add rpc_legacy example to book. Add missing light_client example too

* cargo fmt

---------

Co-authored-by: Fredrik Simonsson <fredrik@nodle.com>

subxt/examples/light_client_tx_basic.rs

@@ -0,0 +1,48 @@
+use subxt::{client::LightClient, PolkadotConfig};
+use subxt_signer::sr25519::dev;
+
+// Generate an interface that we can use from the node's metadata.
+#[subxt::subxt(runtime_metadata_path = "../artifacts/polkadot_metadata_small.scale")]
+pub mod polkadot {}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // The smoldot logs are informative:
+    tracing_subscriber::fmt::init();
+
+    // Create a light client by fetching the chain spec of a local running node.
+    // In this case, because we start one single node, the bootnodes must be overwritten
+    // for the light client to connect to the local node.
+    //
+    // The `12D3KooWEyoppNCUx8Yx66oV9fJnriXwCcXwDDUA2kj6vnc6iDEp` is the P2P address
+    // from a local polkadot node starting with
+    // `--node-key 0000000000000000000000000000000000000000000000000000000000000001`
+    let api = LightClient::<PolkadotConfig>::builder()
+        .bootnodes([
+            "/ip4/127.0.0.1/tcp/30333/p2p/12D3KooWEyoppNCUx8Yx66oV9fJnriXwCcXwDDUA2kj6vnc6iDEp",
+        ])
+        .build_from_url("ws://127.0.0.1:9944")
+        .await?;
+
+    // Build a balance transfer extrinsic.
+    let dest = dev::bob().public_key().into();
+    let balance_transfer_tx = polkadot::tx().balances().transfer_allow_death(dest, 10_000);
+
+    // Submit the balance transfer extrinsic from Alice, and wait for it to be successful
+    // and in a finalized block. We get back the extrinsic events if all is well.
+    let from = dev::alice();
+    let events = api
+        .tx()
+        .sign_and_submit_then_watch_default(&balance_transfer_tx, &from)
+        .await?
+        .wait_for_finalized_success()
+        .await?;
+
+    // Find a Transfer event and print it.
+    let transfer_event = events.find_first::<polkadot::balances::events::Transfer>()?;
+    if let Some(event) = transfer_event {
+        println!("Balance transfer success: {event:?}");
+    }
+
+    Ok(())
+}