Add async-std support to smtp transport (#531)

* Add async-std support to smtp transport

* fix Tls import

* fix copy paste

* too many imports

* Temporarely skip async-std native-tls support

* Fix panic message

* TlsParameters: use rustls when async-std is enabled

examples/asyncstd1_smtp_tls.rs

@@ -0,0 +1,31 @@
+use lettre::{
+    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Connector,
+    AsyncStd1Transport, Message,
+};
+
+#[async_std::main]
+async fn main() {
+    tracing_subscriber::fmt::init();
+
+    let email = Message::builder()
+        .from("NoBody <nobody@domain.tld>".parse().unwrap())
+        .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
+        .to("Hei <hei@domain.tld>".parse().unwrap())
+        .subject("Happy new async year")
+        .body(String::from("Be happy with async!"))
+        .unwrap();
+
+    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+
+    // Open a remote connection to gmail
+    let mailer = AsyncSmtpTransport::<AsyncStd1Connector>::relay("smtp.gmail.com")
+        .unwrap()
+        .credentials(creds)
+        .build();
+
+    // Send the email
+    match mailer.send(email).await {
+        Ok(_) => println!("Email sent successfully!"),
+        Err(e) => panic!("Could not send email: {:?}", e),
+    }
+}