@spec association_type(module(), atom()) ::
  :belongs_to | :has_many | :has_one | :many_to_many | nil

Get the association type for a field in a schema.

Returns:

• :belongs_to for belongs_to associations
• :has_many for has_many associations
• :has_one for has_one associations
• :many_to_many for many_to_many associations
• nil if the field is not an association

@spec foreign_key(module(), atom()) :: atom() | nil

Get the foreign key for a has_many or has_one association.