docs: add TOFU certificate validation example

Add examples/tofu.rs, demonstrating Trust On First Use (TOFU)
certificate validation for SSL connections.

The example shows how to implement a custom TofuStore and how to
configure a client to use TOFU for secure certificate verification.
It uses a simple in-memory TofuStore implementation for
demonstration purposes only.

Author: lucasdbr05

examples/tofu.rs

@@ -0,0 +1,43 @@
+extern crate electrum_client;
+
+use electrum_client::{Client, ConfigBuilder, ElectrumApi, TofuStore};
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex};
+
+/// A simple in-memory implementation of TofuStore for demonstration purposes.
+#[derive(Debug, Default)]
+struct MyTofuStore {
+    certs: Mutex<HashMap<String, Vec<u8>>>,
+}
+
+impl TofuStore for MyTofuStore {
+    fn get_certificate(
+        &self,
+        host: &str,
+    ) -> Result<Option<Vec<u8>>, Box<dyn std::error::Error + Send + Sync>> {
+        let certs = self.certs.lock().unwrap();
+        Ok(certs.get(host).cloned())
+    }
+
+    fn set_certificate(
+        &self,
+        host: &str,
+        cert: Vec<u8>,
+    ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+        let mut certs = self.certs.lock().unwrap();
+        certs.insert(host.to_string(), cert);
+        Ok(())
+    }
+}
+
+fn main() {
+    let store = Arc::new(MyTofuStore::default());
+    let config = ConfigBuilder::new().tofu_store(store).build();
+
+    let client =
+        Client::from_config("ssl://electrum.blockstream.info:50002", config).unwrap();
+    let res = client.server_features();
+    println!("{:#?}", res);
+}
+
+