[20817] Add CDRMessage_t header

[elianalf]

Description

This PR adds CDRMessage_t.h to ShapesDemo to fix a compilation error introduced from the related implementation PR:

• eProsima/Fast-DDS#4701

Contributor Checklist

• [x] Commit messages follow the project guidelines.
• [x] Changes do not break current interoperability.
• N/A New feature has been documented/Current behavior is correctly described in the documentation.
• N/A Applicable backports have been included in the description.

Reviewer Checklist

• [ ] The PR has a milestone assigned.
• [ ] The title and description correctly express the PR's purpose.
• [ ] Check contributor checklist is correct.
• [ ] CI passes without warnings or errors.