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/mimefactory/mimefactory_tests.rs

@@ -832,8 +832,8 @@ async fn test_protected_headers_directive() -> Result<()> {
             .count(),
         1
     );
-    assert_eq!(part.match_indices("Subject:").count(), 1);
-
+    assert_eq!(part.match_indices("Subject:").count(), 2);
+    assert_eq!(part.match_indices("HP-Outer: Subject:").count(), 1);
     Ok(())
 }