Allow to customize foreign types in `#[derive(Associations)]`
Allow to customize foreign types in `#[derive(Associations)]`

This commit enables to usage of wrapper types for foreign keys in
`#[derive(Associations)]`
Basically those wrapper types must fulfil the following condition:
`&'a FieldType: Into<Option<&'a <ParentStruct as Identifiable>::Id>>`

diesel-rs/diesel,diesel-rs/diesel,sgrif/diesel,sgrif/diesel