[ER Diagrams] Add ER diagram APIs and sample data

[OnkarVO7]

Describe your changes:

Add ER diagram APIs and sample data

Type of change:

• [ ] Bug fix
• [ ] Improvement
• [s] New feature
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation

Checklist:

• [x] I have read the CONTRIBUTING document.
• [ ] My PR title is Fixes <issue-number>: <short explanation>
• [ ] I have commented on my code, particularly in hard-to-understand areas.
• [ ] For JSON Schema changes: I updated the migration scripts or explained why it is not needed.