tokio-xmpp: explicitly set default CryptoProvider in all examples

This way, we don't end up with breakage if rustls is built with more
than one provider enabled. This can (and does) happen if a transitive
dependency (such as reqwest *hinthint*) depends on a specific,
non-default rustls backend.

tokio-xmpp/examples/echo_server.rs

@@ -4,11 +4,17 @@ use tokio::{self, io, net::TcpSocket};
 use tokio_xmpp::{
     minidom::Element,
     parsers::stream_features::StreamFeatures,
+    rustls,
     xmlstream::{accept_stream, StreamHeader, Timeouts},
 };
 
 #[tokio::main]
 async fn main() -> Result<(), io::Error> {
+    #[cfg(feature = "tls-rust")]
+    rustls::crypto::aws_lc_rs::default_provider()
+        .install_default()
+        .expect("failed to install rustls crypto provider");
+
     // TCP socket
     let address = "127.0.0.1:5222".parse().unwrap();
     let socket = TcpSocket::new_v4()?;