🚀 Feature: Relationship - unique index

### 🔖 Feature description Allow to include the id of a relationship (many to one for example) in a unique index. ### 🎤 Pitch Useful for a lot of use cases. For example: An order can't have same product in two lines, so i need unique index (product_id, order_id) collection: order_line - attr: product_id (relationship) - attr: order_id (relationship) ### 👀 Have you spent some time to check if this issue has been raised before? - [X] I checked and didn't find similar issue ### 🏢 Have you read the Code of Conduct? - [X] I have read the [Code of Conduct](https://github.com/appwrite/.github/blob/main/CODE_OF_CONDUCT.md)