Add a required_callback to inject user defined requirement checks

include/CLI/App.hpp

@@ -146,6 +146,9 @@ class App {
     /// This is a function that runs when all processing has completed
     std::function<void()> final_callback_{};
 
+    /// This is a function tat runs after all standard requirement checks havv been processed
+    std::function<void()> require_callback_{};
+
     ///@}
     /// @name Options
     ///@{
@@ -376,6 +379,13 @@ class App {
         return this;
     }
 
+    /// Set a callback to execute after all standard requirement checks have been processed
+    ///
+    App *require_callback(std::function<void()> req_callback) {
+        require_callback_ = std::move(req_callback);
+        return this;
+    }
+
     /// Set a name for the app (empty will use parser to set the name)
     App *name(std::string app_name = "");
 

include/CLI/impl/App_inl.hpp

@@ -1410,6 +1410,10 @@ CLI11_INLINE void App::_process_requirements() {
             throw(CLI::RequiredError(sub->get_display_name()));
         }
     }
+
+    if (require_callback_) {
+        require_callback_();
+    }
 }
 
 CLI11_INLINE void App::_process() {