Support union discriminator's @default annotation

[richiware]

Description

This PR adds support for union discriminator's @default annotation.

Depends on:

• New case for default union discriminator value [21235] dds-types-test#32
• Annotations for union's discriminators [21236] IDL-Parser#148

Contributor Checklist

• Commit messages follow the project guidelines.
• 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 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.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.