[#16] Reorganization and refactoring

[orecham]

Notes for Reviewer

1. Clearly separate code for:
   1. RMW API
   2. Implementation details
2. Naming convention to distinguish RMW and Impl types
3. Fix naming of BasicTypes demo nodes

Pre-Review Checklist for the PR Author

• Add sensible notes for the reviewer
• PR title is short, expressive and meaningful
• Relevant issues are linked in the References section
• Every source code file has a copyright header with SPDX-License-Identifier: Apache-2.0 OR MIT
• Branch follows the naming format (rmw-iox2-123-implement-graph-api)
• Commit messages have the issue ID ([#123] Add posix ipc example)
• Tests exist for new behavior
• Changelog updated in the unreleased section including API breaking changes
• Assign PR to reviewer

Checklist for the PR Reviewer

• Commits are properly organized and messages are according to the guideline
• Unit tests have been written for new behavior
• Public API is documented
• PR title describes the changes

Post-review Checklist for the PR Author

• All open points are addressed and tracked via issues

References

Closes #16