Support for composite Primary Keys

[ElijahAhianyo]

For now there's only support for a single field as a primary key. Would be nice to have a composite_primary_key attribute as well. Ideally, this should inherently create a logical key(eg. pk) that is a tuple of the composites.

#[model]
struct Bar{
   #[model(primary_key)]
   id: Auto<u32>
}

#[model]
struct Baz{
    #[model(primary_key)
    id: Auto<u32>
} 

#[model]
struct Foo{
   #[model(composite_key)]
   bar: ForeignKey<Bar>,
   #[model(composite_key)]
   baz: ForeignKey<Baz>,
}