Add pivot class property

[d3v2a]

Summary

This PR add pivot property when using a pivot model on BelongsToMany relation

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Misc. change (internal, infrastructure, maintenance, etc.)

Checklist

• [ ] Existing tests have been adapted and/or new tests have been added
• [ ] Add a CHANGELOG.md entry
• [ ] Update the README.md
• [x] Code style has been fixed via composer fix-style

[barryvdh]

Hmm two tests are failing

[d3v2a]

@barryvdh Is there anything more to do?