fix: types for model API with compound unique and findOne

[jonbretman]

This fixes the generation of the unique conditions interface for relationship fields.

As an example the following schema:

model Profile {
    fields {
        name Text
    }
}

model Follow {
    fields {
        from Profile
        to Profile
    }

    @unique([from, to])
}

Currently produces a unique conditions interface for Follow that looks like this. Both not grouping the from and to fields into one object, but also using the whole Profile model as the type.

export type FollowUniqueConditions = 
    | {from: Profile}
    | {to: Profile}
    | {id: string};

With this fix the unique conditions interface is now this, which correctly groups the fields and uses the id's rather than the whole model.

export type FollowUniqueConditions = 
    | {fromId: string, toId: string}
    | {id: string};