Add @optional support to `rosidl_generator_cpp`

[InvincibleRMC]

Description

In Draft state since there are downstream rmw problems (tested with fastrps). I assume similar problems will occur with other rmw implementations.

For example calling .size() on the const class std::optional<std::__cxx11::basic_string<char> >

// Member: optional_string
  current_alignment += padding +
    eprosima::fastcdr::Cdr::alignment(current_alignment, padding) +
    (ros_message.optional_string.size() + 1);

A similar problem occurs with containers.

Fixes #850

Is this user-facing behavior change?

Since this adds optionals users will not have any breaking changes.

Did you use Generative AI?

Additional Information