[21785] Fix encoding/decoding when inner structure has different extensibility

[richiware]

Description

This PR fixes encoding/decoding of a structure with an inner structure with different extensibility.

@appendable
struct InnerAppendableStructure
{
    @id(3)
    unsigned long value1;
    @id(16) @optional
    unsigned long value2;
 };

@mutable
struct MutableWithInnerAppendableStruct
{
   @id(1)
   unsigned long value1;
   @id(2)
   InnerAppendableStructure value2;
};

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