Prepare async index for auto_canceled_by_id_convert_to_bigint

As titled, this is the first step of creating indexes asynchronously.
See guideline at https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#schedule-the-index-to-be-created

It's to schedule the index creation for
`p_ci_builds.auto_canceled_by_id_convert_to_bigint`
as the foreign key constraint requires the index
so that we can later swap `auto_canceled_by_id_convert_to_bigint` with `auto_canceled_by_id`

Changelog: added

db/post_migrate/20240122071840_ensure_backfill_for_ci_builds_integer_columns_is_finished.rb

+# frozen_string_literal: true
+
+class EnsureBackfillForCiBuildsIntegerColumnsIsFinished < Gitlab::Database::Migration[2.2]
+  include Gitlab::Database::MigrationHelpers::ConvertToBigint
+  milestone '16.9'
+
+  restrict_gitlab_migration gitlab_schema: :gitlab_ci
+  disable_ddl_transaction!
+
+  TABLE_NAME = :ci_builds
+  COLUMN_NAMES = %w[
+    auto_canceled_by_id
+    commit_id
+    erased_by_id
+    project_id
+    runner_id
+    trigger_request_id
+    upstream_pipeline_id
+    user_id
+  ]
+  BIGINT_COLUMN_NAMES = COLUMN_NAMES.map { |name| "#{name}_convert_to_bigint" }
+
+  def up
+    ensure_batched_background_migration_is_finished(
+      job_class_name: 'CopyColumnUsingBackgroundMigrationJob',
+      table_name: TABLE_NAME,
+      column_name: 'id',
+      job_arguments: [COLUMN_NAMES, BIGINT_COLUMN_NAMES]
+    )
+  end
+
+  def down
+    # no-op
+  end
+end

db/post_migrate/20240123071840_prepare_async_indexes_for_p_ci_builds_auto_canceled_by_id.rb

+# frozen_string_literal: true
+
+class PrepareAsyncIndexesForPCiBuildsAutoCanceledById < Gitlab::Database::Migration[2.2]
+  include Gitlab::Database::PartitioningMigrationHelpers
+
+  milestone '16.9'
+  disable_ddl_transaction!
+
+  INDEX_NAME = "p_ci_builds_auto_canceled_by_id_bigint_idx"
+  TABLE_NAME = :p_ci_builds
+  COLUMN_NAME = :auto_canceled_by_id_convert_to_bigint
+  WHERE_CLAUSE = "auto_canceled_by_id_convert_to_bigint IS NOT NULL"
+
+  def up
+    Gitlab::Database::PostgresPartitionedTable.each_partition(TABLE_NAME) do |partition|
+      index_name = generated_index_name(partition.identifier, INDEX_NAME)
+      prepare_async_index partition.identifier, COLUMN_NAME, name: index_name, where: WHERE_CLAUSE
+    end
+  end
+
+  def down
+    Gitlab::Database::PostgresPartitionedTable.each_partition(TABLE_NAME) do |partition|
+      index_name = generated_index_name(partition.identifier, INDEX_NAME)
+      unprepare_async_index partition.identifier, COLUMN_NAME, name: index_name, where: WHERE_CLAUSE
+    end
+  end
+end

db/post_migrate/20240124081840_prepare_async_indexes_for_p_ci_builds_commit_id_part1.rb

+# frozen_string_literal: true
+
+class PrepareAsyncIndexesForPCiBuildsCommitIdPart1 < Gitlab::Database::Migration[2.2]
+  include Gitlab::Database::PartitioningMigrationHelpers
+
+  milestone '16.9'
+  disable_ddl_transaction!
+
+  INDEXES = [
+    [[:commit_id_convert_to_bigint, :status, :type], "p_ci_builds_commit_id_bigint_status_type_idx"],
+    [[:commit_id_convert_to_bigint, :type, :name, :ref], "p_ci_builds_commit_id_bigint_type_name_ref_idx"]
+  ]
+  TABLE_NAME = :p_ci_builds
+
+  def up
+    Gitlab::Database::PostgresPartitionedTable.each_partition(TABLE_NAME) do |partition|
+      INDEXES.each do |columns, routing_table_index_name|
+        index_name = generated_index_name(partition.identifier, routing_table_index_name)
+        prepare_async_index partition.identifier, columns, name: index_name
+      end
+    end
+  end
+
+  def down
+    Gitlab::Database::PostgresPartitionedTable.each_partition(TABLE_NAME) do |partition|
+      INDEXES.each do |columns, routing_table_index_name|
+        index_name = generated_index_name(partition.identifier, routing_table_index_name)
+        unprepare_async_index partition.identifier, columns, name: index_name
+      end
+    end
+  end
+end

db/schema_migrations/20240122071840

+a404dcf1d57886af351b696f8cbc451e7e81d69618a6bd3c30e1676a99172ca6
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db/schema_migrations/20240123071840

+3feac709d7c1e6d911068d096e876fcbdace5c3e47036a3c8d9bb347a4d6f4ba
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db/schema_migrations/20240124081840

+93164c045f7581f7dbcd606f2217514344bf2fdb72a6ece05241214e7a7f3c9d
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