Supply metadata.content_type for to html output (#14990)

# Description

Adds pipeline metadata to the `to html` command output (hardcoded to
`text/html; charset=utf-8`)

# User-Facing Changes

Pipeline metadata is now included with the `to html` command output.

crates/nu-command/tests/format_conversions/html.rs

@@ -14,6 +14,15 @@ fn out_html_simple() {
     );
 }
 
+#[test]
+fn out_html_metadata() {
+    let actual = nu!(r#"
+            echo 3 | to html | metadata | get content_type
+        "#);
+
+    assert_eq!(actual.out, r#"text/html; charset=utf-8"#);
+}
+
 #[test]
 fn out_html_partial() {
     let actual = nu!(r#"