[21052] Make ChangeForReader, ReaderLocator, ReaderProxy private

[Mario-DL]

Description

This PR makes ChangeForReader, ReaderLocator, ReaderProxy private and reorganizes the header in the affected source files.

Contributor Checklist

• [X] Commit messages follow the project guidelines.
• [X] The code follows the style guidelines of this project.
• NO 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
• NO Any new/modified methods have been properly documented using Doxygen.
• NO 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.
• [X] New feature has been added to the versions.md file (if applicable).
• NO New feature has been documented/Current behavior is correctly described in the documentation.
• NO 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.