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

[InvincibleRMC]

I believe this is as far this PR can go without updating each of the dds rosidl_typesupport_**_cpp. I'm not sure if it is better to merge this and then update them. Update them then merge this. Or try and so some simultaneous merge. The only downstream change would effect if people had already added the @optional annotation.

[InvincibleRMC]

Found a bug in container type_traits working on a fix.