Description

This PR migrates all the fastrtps namespace to fastdds. It also removes all the unnecessary nested namespace and using namespace.

Related PR:
Fast DDS Python: eProsima/Fast-DDS-python#135
Shapes Demo: eProsima/ShapesDemo#141
Fast DDS GEN: eProsima/Fast-DDS-Gen#350
Discovery Server: eProsima/Discovery-Server#87

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• N/A 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 backport compatible: they do NOT break ABI nor change library core behavior.
• ❌ Changes are API compatible.
• 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: [21129] Migrate fastrtps namespace Fast-DDS-docs#813
• 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.