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/Services/WebContent/WebContentServer.ipc

@@ -39,6 +39,7 @@ endpoint WebContentServer
 
     get_document_element() => (Optional<i32> node_id)
     query_selector_all(i32 start_node_id, String selector) => (Optional<Vector<i32>> elements_ids)
+    get_element_attribute(i32 element_id, String name) => (Optional<String> attribute)
 
     run_javascript(String js_source) =|