Add a way to persist a calculation for a resource

[Dak425]

Is your feature request related to a problem? Please describe.
When defining generated attributes for a resource, I often see people needing to manually edit the migration to use ALTER TABLE to explicitly add the column with the SQL statement used to populate the column.

Describe the solution you'd like
A way to in an Ash resource to define an attribute that is generated, specify the raw SQL statement to populate the field, and have the generated migration use the Ecto syntax for configuring such a column shown here

Describe alternatives you've considered
Continue manually changing migrations or replace generated attribute with calculations that are not persisted in a column

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

Idea 1 - generate_with attribute option

Using calculation

  attributes do
    attribute :generated_foo, :string, generate_with: :full_name
  end

  calculations do
    calculate :full_name, :string, expr(first_name <> " " <> last_name)
  end

Using raw SQL

  attributes do
    attribute :generated_foo, :string, generate_with: expr(fragment("? || ' ' || ?", first_name, last_name))
  end

Idea 2 - persist calculation option

  calculations do
    calculate :full_name, :string, expr(first_name <> " " <> last_name)

    persist? true
  end

Additional context
Link to the start of the conversation in the Elixir Discord server that lead to this proposal: message