[20765] Make RTPS reader implementations private

[elianalf]

Description

This PR makes RTPS reader implementations private:

• StatefulPersistentReader
• StatefulReader
• StatelessPersistentReader
• StatelessReader

Related PR in Fast DDS Python:

• [20765] Remove rtps reader implemantation made private Fast-DDS-python#120

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 ABI compatible.
• ❌ Changes are API compatible.
• N/A New feature has been added to the versions.md file (if applicable).
• 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.

[MiguelCompany]

@elianalf There are conflicts here, so a rebase is necessary.

[MiguelCompany]

It would be nice to also change the extension of the moved headers from .h to `.hpp

[elianalf]

@richiprosima please test_3 this