Add migration to backfill data

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We currently have a migration to create a collection and its partitions. Next we need a migration to backfill data.

From the blueprint:

class BackfillContentToIssueCollection < Gitlab::Ai::Context::Migration[1.0]
  milestone '18.0'

  def change
    backfill_collection :issues, field: :content
  end
end

Ref

We probably also want to backfill an entire collection.

Definition of done:

• it's possible to create a migration to backfill a single field
• it's possible to create a migration to backfill a full collection
• backfills are batched to prevent adding system strain