[cxx]: GPIO CXX wrappers, experiemental CI rule

* Wrapper class for simple GPIO interaction
  like read/write without ISRs.
* Added rule to provoke builds after changes in
  the experimental C++ component.

examples/cxx/experimental/experimental_cpp_component/include/system_cxx.hpp

@@ -0,0 +1,53 @@
+/*
+ * SPDX-FileCopyrightText: 2021 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#pragma once
+
+#ifndef __cpp_exceptions
+#error system C++ classes only usable when C++ exceptions enabled. Enable CONFIG_COMPILER_CXX_EXCEPTIONS in Kconfig
+#endif
+
+/**
+ * This is a "Strong Value Type" base class for types in IDF C++ classes.
+ * The idea is that subclasses completely check the contained value during construction.
+ * After that, it's trapped and encapsulated inside and cannot be changed anymore.
+ * Consequently, the API functions receiving a correctly implemented sub class as parameter
+ * don't need to check it anymore. Only at API boundaries the valid value will be retrieved
+ * with get_value().
+ */
+template<typename ValueT>
+class StrongValue {
+protected:
+    StrongValue(ValueT value_arg) : value(value_arg) { }
+
+    ValueT get_value() const {
+        return value;
+    }
+
+private:
+    ValueT value;
+};
+
+/**
+ * This class adds comparison properties to StrongValue, but no sorting properties.
+ */
+template<typename ValueT>
+class StrongValueComparable : public StrongValue<ValueT> {
+protected:
+    StrongValueComparable(ValueT value_arg) : StrongValue<ValueT>(value_arg) { }
+
+    using StrongValue<ValueT>::get_value;
+
+    bool operator==(const StrongValueComparable<ValueT> &other_gpio) const
+    {
+        return get_value() == other_gpio.get_value();
+    }
+
+    bool operator!=(const StrongValueComparable<ValueT> &other_gpio) const
+    {
+        return get_value() != other_gpio.get_value();
+    }
+};