Add temporary index for pipeline_artifacts_size

What does this MR do and why?

So that we can efficiently query project_statistics#pipeline_artifacts_size during a backfill, this MR adds a temporary index which can be removed once the related backfill has completed

Refs !126053 (merged) and https://gitlab.com/gitlab-org/gitlab/-/issues/412634

Database info

db:migrate

bin/rails db:migrate

main: == [advisory_lock_connection] object_id: 223980, pg_backend_pid: 59489
main: == 20230718145613 AddTempIndexForProjectStatisticsPipelineArtifactsSizeMigration: migrating
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0821s
main: -- index_exists?(:project_statistics, [:project_id], {:name=>"tmp_index_project_statistics_pipeline_artifacts_size", :where=>"pipeline_artifacts_size <> 0", :algorithm=>:concurrently})
main:    -> 0.0041s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- add_index(:project_statistics, [:project_id], {:name=>"tmp_index_project_statistics_pipeline_artifacts_size", :where=>"pipeline_artifacts_size <> 0", :algorithm=>:concurrently})
main:    -> 0.0042s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20230718145613 AddTempIndexForProjectStatisticsPipelineArtifactsSizeMigration: migrated (0.1055s)

main: == [advisory_lock_connection] object_id: 223980, pg_backend_pid: 59489

db:rollback

bin/rails db:rollback:main
main: == [advisory_lock_connection] object_id: 223460, pg_backend_pid: 60089
main: == 20230718145613 AddTempIndexForProjectStatisticsPipelineArtifactsSizeMigration: reverting
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0845s
main: -- indexes(:project_statistics)
main:    -> 0.0039s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- remove_index(:project_statistics, {:algorithm=>:concurrently, :name=>"tmp_index_project_statistics_pipeline_artifacts_size"})
main:    -> 0.0014s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20230718145613 AddTempIndexForProjectStatisticsPipelineArtifactsSizeMigration: reverted (0.1019s)

Cold query before migration (4 mins): https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/20495/commands/67128

Warm query before migration (3 secs): https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/20495/commands/67129

Warm query after migration (323ms): https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/20495/commands/67131

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