Support Updating of Slug

[allenwyma]

Is your feature request related to a problem? Please describe.
When a value is updated, the slug doesn't seem to update.

Describe the solution you'd like
The function should apply whenever the value has been updated.

It'd also be nice to have a "callback" where we can keep track of the last value, and we can say insert that into a table and redirect old slugs to the new slug for bookmarked pages, etc.

Describe alternatives you've considered
This may be possible to be done as is, but I htink having this in the DSL, would be better to have as it's more clear and realted to ash_slug.

Express the feature either with a change to resource syntax, or with a change to the resource interface

Existing:

  actions do
    update :update do
      change slugify(:name, into: :slug)
    end
  end

To

 ash_slug do
  slugify(:name, into: slug), on: [:create, :update] do
     change fn changeset ->
     # do what you want with the old value, etc.
     end
  end
 end

Additional context
Add any other context or screenshots about the feature request here.