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/download_avatars.rs

@@ -4,7 +4,7 @@ use std::fs::{create_dir_all, File};
 use std::io::{self, Write};
 use std::process::exit;
 use std::str::FromStr;
-use tokio_xmpp::{Client, Stanza};
+use tokio_xmpp::{rustls, Client, Stanza};
 use xmpp_parsers::{
     avatar::{Data as AvatarData, Metadata as AvatarMetadata},
     caps::{compute_disco, hash_caps, Caps},
@@ -26,6 +26,11 @@ use xmpp_parsers::{
 async fn main() {
     env_logger::init();
 
+    #[cfg(feature = "tls-rust")]
+    rustls::crypto::aws_lc_rs::default_provider()
+        .install_default()
+        .expect("failed to install rustls crypto provider");
+
     let args: Vec<String> = args().collect();
     if args.len() != 3 {
         println!("Usage: {} <jid> <password>", args[0]);