feat: Add Config::StdHeaderProtectionComposing (enables composing as defined in RFC 9788) (#7130)

And enable it by default as the standard Header Protection is backward-compatible.

Also this tests extra IMF header removal when a message has standard Header Protection since now we
can send such messages.

src/config.rs

@@ -445,6 +445,12 @@ pub enum Config {
 
     /// Return an error from `receive_imf_inner()` for a fully downloaded message. For tests.
     FailOnReceivingFullMsg,
+
+    /// Enable composing emails with Header Protection as defined in
+    /// <https://www.rfc-editor.org/rfc/rfc9788.html> "Header Protection for Cryptographically
+    /// Protected Email".
+    #[strum(props(default = "1"))]
+    StdHeaderProtectionComposing,
 }
 
 impl Config {