message: export has_convert_from_string

Commit 87b6a8d331867c24f7bd5f0160199478d97383b0 added a SFINAE
template to determine if a type has 'convert_from_string'.
Export this template out of the details namespace so other code
can use it.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Idf8b95b4b76854c7d94729add9101c978ee6056b
diff --git a/include/sdbusplus/message/native_types.hpp b/include/sdbusplus/message/native_types.hpp
index c5bb54a..3aa6dd5 100644
--- a/include/sdbusplus/message/native_types.hpp
+++ b/include/sdbusplus/message/native_types.hpp
@@ -296,6 +296,11 @@
 
 } // namespace details
 
+/** Export template helper to determine if a type has convert_from_string. */
+template <typename T>
+inline constexpr bool has_convert_from_string_v =
+    details::has_convert_from_string_v<T>;
+
 } // namespace message
 } // namespace sdbusplus