[21795] CdrSizeCalculator adds 4 bytes extra in a fixed string member of mutable structure in XCDRv2

[richiware]

Description

It was detected that CdrSizeCalculator adds 4 bytes extra when calculating a fixed string member of a @mutable structure in XCdrv2. For example:

@mutable
struct FixedStringMember
{
    string<20> var;
};

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A Any new/modified methods have been properly documented using Doxygen.
• Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• Changes are API compatible.
• N/A New feature has been added to the versions.md file (if applicable).
• 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.
• Check CI results: changes do not issue any warning.
• Check CI results: CI pass and failing tests are unrelated with the changes.

[MiguelCompany]

LGTM with green CI