[20733] Fix support for @key annotation in Dynamic types (backport #4694)

[mergify]

Description

This PR:

• Fixes various things regarding the handling of @key annotation on dynamic types:
  • Copy the type annotations when creating MemberDescriptors
  • Fix AnnotationDescriptor::key_annotation() implementation
  • Fix population of is_key_defined_ in DynamicType::copy_from_builder()
  • Simplify DynamicType::key_annotation() implementation
• On loading types from XML, add the @key annotation only to the member builder and not to the dynamic type, as @key only makes sense for members but not types.
• Adds the key attribute to members in XSD

@Mergifyio backport 2.13.x 2.10.x 2.6.x

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.
• N/A: Any new configuration API has an equivalent XML API (with the corresponding XSD extension)
• Changes are ABI compatible.
• Changes are API compatible.
• N/A: New feature has been added to the versions.md file (if applicable).
• New feature has been documented/Current behavior is correctly described in the documentation.
  • Related documentation PR: [20733] Document how to annotate members as key in XML types Fast-DDS-docs#746
• 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: failing tests are unrelated with the changes.

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This is an automatic backport of pull request #4694 done by [Mergify](https://mergify.com).

[JesusPoderoso]

LGTM with green CI

[JesusPoderoso]

@richiprosima please test this