This sets a precident for how we handle `attribute` calls, which aren't
backed by a database column. We should not take this as a conscious
decision on how to handle them, and this can change when we make
`attribute` public if we have better ideas in the future.

As the composed attributes API gets fleshed out, I expect the
`persistable_attributes` method to change to
`@attributes.select(&:persistable).keys`, or some more performant
variant there-of. This can probably go away completely once we fully
move dirty checking into the attribute objects once it gets moved up to
Active Model.

Fixes #18407