Use nightly rustfmt features to improve code style (#734)

* format_code_in_doc_comments

* imports_granularity

* group_imports

* Add ci job

src/transport/smtp/client/net.rs

@@ -9,7 +9,6 @@ use std::{
 
 #[cfg(feature = "native-tls")]
 use native_tls::TlsStream;
-
 #[cfg(feature = "rustls-tls")]
 use rustls::{ClientConnection, ServerName, StreamOwned};