Rework light client (#1475)

* WIP second pass over light client code for simpler API

* First pass new light client

* pub(crate) LightClientRpc::new_raw(), and fmt

* Update examples and add back a way to configure boot nodes and fetch chainspec from a URL

* Fix light client examples

* remove unused deps and tidy lightclient feature flags

* fix wasm error

* LightClientRpc can be cloned

* update light client tests

* Other small fixes

* exclude mod unless jsonrpsee

* Fix wasm-lightclient-tests

* add back docsrs bit and web+native feature flag compile error

* update book and light client example names

* fix docs

subxt/examples/light_client_basic.rs

@@ -0,0 +1,47 @@
+#![allow(missing_docs)]
+use futures::StreamExt;
+use subxt::{client::OnlineClient, lightclient::LightClient, PolkadotConfig};
+
+// 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 {}
+
+const POLKADOT_SPEC: &str = include_str!("../../artifacts/demo_chain_specs/polkadot.json");
+const ASSET_HUB_SPEC: &str =
+    include_str!("../../artifacts/demo_chain_specs/polkadot_asset_hub.json");
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // The lightclient logs are informative:
+    tracing_subscriber::fmt::init();
+
+    // Instantiate a light client with the Polkadot relay chain,
+    // and connect it to Asset Hub, too.
+    let (lightclient, polkadot_rpc) = LightClient::relay_chain(POLKADOT_SPEC)?;
+    let asset_hub_rpc = lightclient.parachain(ASSET_HUB_SPEC)?;
+
+    // Create Subxt clients from these Smoldot backed RPC clients.
+    let polkadot_api = OnlineClient::<PolkadotConfig>::from_rpc_client(polkadot_rpc).await?;
+    let asset_hub_api = OnlineClient::<PolkadotConfig>::from_rpc_client(asset_hub_rpc).await?;
+
+    // Use them!
+    let polkadot_sub = polkadot_api
+        .blocks()
+        .subscribe_finalized()
+        .await?
+        .map(|block| ("Polkadot", block));
+    let parachain_sub = asset_hub_api
+        .blocks()
+        .subscribe_finalized()
+        .await?
+        .map(|block| ("AssetHub", block));
+
+    let mut stream_combinator = futures::stream::select(polkadot_sub, parachain_sub);
+
+    while let Some((chain, block)) = stream_combinator.next().await {
+        let block = block?;
+        println!("     Chain {:?} hash={:?}", chain, block.hash());
+    }
+
+    Ok(())
+}