fix: add missing c-tor for nesting Variants (#467)

This allows nesting variants, i.e. allows a variant to contain another variant as its value. Until now, there was no such possibility, since the default generated copy constructor would be invoked which would create a copy of source variant instead of embed the source variant as a value in the destination variant. The default generated copy constructor is kept, for it makes sense too, but a new tag-based overload is added for embedding the source variant into the destination variant.

include/sdbus-c++/TypeTraits.h

@@ -106,6 +106,9 @@ namespace sdbus {
     // Tag denoting a call where the reply shouldn't be waited for
     struct dont_expect_reply_t { explicit dont_expect_reply_t() = default; };
     inline constexpr dont_expect_reply_t dont_expect_reply{};
+    // Tag denoting that the variant shall embed the other variant as its value, instead of creating a copy
+    struct embed_variant_t { explicit embed_variant_t() = default; };
+    inline constexpr embed_variant_t embed_variant{};
 
     // Helper for static assert
     template <class... _T> constexpr bool always_false = false;

include/sdbus-c++/Types.h

@@ -67,6 +67,14 @@ namespace sdbus {
             msg_.seal();
         }
 
+        Variant(const Variant& value, embed_variant_t) : Variant()
+        {
+            msg_.openVariant<Variant>();
+            msg_ << value;
+            msg_.closeVariant();
+            msg_.seal();
+        }
+
         template <typename _Struct>
         explicit Variant(const as_dictionary<_Struct>& value) : Variant()
         {