varchar size info for a column that already exists is missing from migration

Describe the bug Adding a specific size for a string/varchar field will not show in the migration file if the column already exists from another migration.

To Reproduce First create the following resource:

defmodule MyResource do
  use Ash.Resource, data_layer: AshPostgres.DataLayer
  
  attributes do
    uuid_primary_key :id
    
    attribute :blibs, :string
  end
  
  postgres do
    table "my_resources"

    repo Repo
  end
end

Now, generate a migration for it with mix ash_postgres.generate_migrations.

This should generate the table my_resources with the blibs field as a :text field.

Now, change the resource to this:

defmodule MyResource do
  use Ash.Resource, data_layer: AshPostgres.DataLayer
  
  attributes do
    uuid_primary_key :id
    
    attribute :blibs, :string
  end
  
  postgres do
    table "my_resources"
    
    migration_types blibs: {:varchar, 255}

    repo Repo
  end
end

And generate a new migration with mix ash_postgres.generate_migrations

This will generate the following migration:

  def up do
    alter table(:my_resources) do
      modify :blibs, :varchar
    end
  end

  def down do
    alter table(:my_resources) do
      modify :blibs, :text
    end
  end

Expected behavior The final generated migration in the example above should contain information about the column size (in this case, it should add , size: 255.

** Runtime

• Elixir version 1.14.4
• Erlang version 25
• OS Fedora 28
• Ash version 2.9.18
• any related extension versions 1.3.28