Prepare async index for p_ci_builds.user_id

What does this MR do and why?

Prepare async index for p_ci_builds.user_id

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

Changelog: added

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Before	After

How to set up and validate locally

1. Run the following commands:

   ./scripts/regenerate-schema
   RAILS_ENV=test bin/rails runner 'Feature.enable :database_async_index_operations'
   RAILS_ENV=test bin/rake gitlab:db:execute_async_index_operations:all

2. Confirm the following indexes are created in TEST environment

   psql gitlabhq_test -h $PATH_TO_POSTGRESQL/gitlab-development-kit/postgresql

   \d ci_builds
    "index_6d0ca9fd9d" btree (user_id_convert_to_bigint)
    "index_837e2982cb" btree (user_id_convert_to_bigint, created_at) WHERE type::text = 'Ci::Build'::text
    "index_a2d2dd4f5f" btree (user_id_convert_to_bigint, name) WHERE type::text = 'Ci::Build'::text AND (name::text = ANY (ARRAY['container_scanning'::character varying::text, 'dast'::character varying::text, 'dependency_scanning'::character varying::text, 'license_management'::character varying::text, 'license_scanning'::character varying::text, 'sast'::character varying::text, 'coverage_fuzzing'::character varying::text, 'secret_detection'::character varying::text]))
    "index_efd693f03f" btree (user_id_convert_to_bigint, name, created_at) WHERE type::text = 'Ci::Build'::text AND (

name::text = ANY (ARRAY['container_scanning'::character varying::text, 'dast'::character varying::text, 'dependency_scanning'::character varying::text, 'license_management'::character varying::text, 'license_scanning'::character varying::text, 'sast'::character varying::text, 'coverage_fuzzing'::character varying::text, 'apifuzzer_fuzz'::character varying::text, 'apifuzzer_fuzz_dnd'::character varying::text, 'secret_detection'::character varying::text]))

3. Run the following commands to restore the feature flag:

```shell
RAILS_ENV=test bin/rails runner 'Feature.disable :database_async_index_operations'

Related to #422835 (closed)