Follow-up to [rails/rails#53954][]
Re-submission of [rails/rails#53887][]
Closes [rails/rails#53793][]
Follow-up to [rails/rails#43945][]

To support this behavior, the bulk of the implementation is moved to the
new `ActiveModel::Normalization` module. Any "persistence"-related
language, methods, and test coverage has been excised. The single
implementation change is related to reading and writing attributes:

```diff
  def normalize_attribute(name)
    # Treat the value as a new, unnormalized value.
-  self[name] = self[name]
+  send(:"#{name}=", send(name))
  end
```

This can be undone if a change like [rails/rails#53886][] lands.

[rails/rails#53954]: https://github.com/rails/rails/pull/53954
[rails/rails#53887]: https://github.com/rails/rails/pull/53887
[rails/rails#53793]: https://github.com/rails/rails/discussions/53793
[rails/rails#43945]: https://github.com/rails/rails/pull/43945
[rails/rails#53886]: https://github.com/rails/rails/pull/53886