PHPORM-114 Implement Builder::upsert

[GromNaN]

Fix PHPORM-114

Laravel documentation for this method: https://laravel.com/docs/11.x/eloquent#upserts

Unlike createOrFirst and firstOrCreate, there is no event triggered on the model by this operation. So we can use MongoDB's bulk write operation to perform multiple upserts in a single command.

Checklist

• Add tests and ensure they pass
• Add an entry to the CHANGELOG.md file
• Update documentation for new features