add frontier/ethereum example

[niklasad1]

The ethereum config doesn't work with the default PolkadotConfig which this shows how to do.

Tested with a frontier node

[jsdw]

Awesome!

I could have sworn that we had a very similar example somewhere, but I couldn't find it offhand so this looks good to me!

[niklasad1]

@CodiumAI-Agent /improve

[CodiumAI-Agent]

PR Code Suggestions

Category	Suggestions
Enhancement	Make the WebSocket URL configurable to enhance flexibility. Replace the hardcoded WebSocket URL with a configurable option, allowing flexibility in specifying different node endpoints. subxt/examples/tx_basic_frontier.rs [38] -let api = OnlineClient::<EthRuntimeConfig>::from_insecure_url("ws://127.0.0.1:9944").await?; +let node_url = std::env::var("NODE_URL").unwrap_or_else(|_| "ws://127.0.0.1:9944".to_string()); +let api = OnlineClient::<EthRuntimeConfig>::from_insecure_url(&node_url).await?;
	Replace println! with log::info! for more structured logging. Use a logging framework instead of println! for better control over logging levels and outputs. subxt/examples/tx_basic_frontier.rs [60] -println!("Balance transfer success: {event:?}"); +log::info!("Balance transfer success: {event:?}");
	Add retry logic to network requests to improve reliability. Consider adding retry logic or a fallback mechanism for network requests to handle transient errors gracefully. subxt/examples/tx_basic_frontier.rs [51-54] -let events = api - .tx() - .sign_and_submit_then_watch_default(&balance_transfer_tx, &alith) - .await?; +let events = retry::retry_async(retry::delay::Exponential::from_millis(10), || async { + api.tx() + .sign_and_submit_then_watch_default(&balance_transfer_tx, &alith) + .await +}).await?;
Possible issue	Add error handling for hex encoding to prevent potential runtime panics. Handle potential errors from the hex::encode function calls to prevent runtime panics in case of encoding failures. subxt/examples/tx_basic_frontier.rs [44-45] -println!("baltathar pub: {}", hex::encode(baltathar.public_key().0)); -println!("baltathar addr: {}", hex::encode(dest)); +println!("baltathar pub: {}", hex::encode(baltathar.public_key().0).unwrap_or_else(|e| e.to_string())); +println!("baltathar addr: {}", hex::encode(dest).unwrap_or_else(|e| e.to_string()));
Best practice	Improve error handling for environment variable retrieval to enhance robustness. Replace the unwrap calls with proper error handling to avoid panics when environment variables are not set or have invalid values. subxt/examples/tx_basic_frontier.rs [38] -let node_url = std::env::var("NODE_URL").unwrap_or_else(|_| "ws://127.0.0.1:9944".to_string()); +let node_url = std::env::var("NODE_URL").unwrap_or_else(|e| { + eprintln!("Error reading NODE_URL: {e}"); + "ws://127.0.0.1:9944".to_string() +});

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[niklasad1]

@CodiumAI-Agent /describe