WebContent+Friends: Add get_element_attribute IPC and plumbing

Author: https://github.com/TobyAsE
Commit: 3f5a620b5d
Pull-request: https://github.com/SerenityOS/serenity/pull/15698
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWebView/OutOfProcessWebView.h

@@ -60,6 +60,7 @@ public:
 
     Optional<i32> get_document_element();
     Optional<Vector<i32>> query_selector_all(i32 start_node_id, String const& selector);
+    Optional<String> get_element_attribute(i32 element_id, String const& name);
 
     void set_content_filters(Vector<String>);
     void set_proxy_mappings(Vector<String> proxies, HashMap<String, size_t> mappings);