Add arbitrary string type to the native object API (#4218)

* Custom strings are very common for optimizations around small objects
  or growth style optimizations, i.e.: grow at 1.57 times vs doubling vs..

  A second common strategy is to cooperate w/ the memory allocator
  see FBString[1] and seastar[2] string for examples.

[1] fbstring: https://github.com/facebook/folly/blob/master/folly/docs/FBString.md
[2] sstring: https://github.com/scylladb/seastar/blob/master/core/sstring.hh

include/flatbuffers/flatbuffers.h

@@ -980,6 +980,15 @@ FLATBUFFERS_FINAL_CLASS
     return str ? CreateString(str->c_str(), str->Length()) : 0;
   }
 
+  /// @brief Store a string in the buffer, which can contain any binary data.
+  /// @param[in] str A const reference to a std::string like type with support
+  /// of T::c_str() and T::length() to store in the buffer.
+  /// @return Returns the offset in the buffer where the string starts.
+  template<typename T>
+    Offset<String> CreateString(const T &str) {
+    return CreateString(str.c_str(), str.length());
+  }
+
   /// @brief Store a string in the buffer, which can contain any binary data.
   /// If a string with this exact contents has already been serialized before,
   /// instead simply returns the offset of the existing string.