[FEATURE] Proper foreignTypes on all inline fields

This change adds:

* An InlineRelationFieldInterface with get/setForeignTypes
* `foreignTypes` (array) argument on `flux:field.inline`
* Transfer of `foreignTypes` to `Inline` component in `flux:field.inline`
* Touch-up added variable type declaration in `flux:field.inline.fal`

Resolves: #495