Add sequence migration helper

Ref: #362984 (closed)

What does this MR do and why?

This MR adds a new migration helper method to drop unused sequences generated by postgres easily.

Why do we need this?

It was brought up in #362984 (closed) that we need to remove existing sequences which are not being used.

This MR simplifies the process to remove them.

Fake migration test

class DropSequenceTest < Gitlab::Database::Migration[2.0]
  def up
    drop_sequence(:ci_pipelines_config, :pipeline_id, :ci_pipelines_config_pipeline_id_seq)
  end

  def down
    add_sequence(:ci_pipelines_config, :pipeline_id, :ci_pipelines_config_pipeline_id_seq, 10)
  end
end

== 20220524102448 DropSequenceTest: migrating ================================
-- quote_table_name(:ci_pipelines_config)
   -> 0.0000s
-- quote_column_name(:pipeline_id)
   -> 0.0000s
-- quote_table_name(:ci_pipelines_config_pipeline_id_seq)
   -> 0.0000s
-- execute("ALTER TABLE \"ci_pipelines_config\" ALTER COLUMN \"pipeline_id\" DROP DEFAULT;\nDROP SEQUENCE IF EXISTS \"ci_pipelines_config_pipeline_id_seq\"\n")
   -> 0.0015s
== 20220524102448 DropSequenceTest: migrated (0.0016s) =======================


--------------- ROLLBACK ---------------------

== 20220524102448 DropSequenceTest: reverting ================================
-- quote_table_name(:ci_pipelines_config)
   -> 0.0000s
-- quote_column_name(:pipeline_id)
   -> 0.0000s
-- execute("CREATE SEQUENCE ci_pipelines_config_pipeline_id_seq START 10;\nALTER TABLE \"ci_pipelines_config\" ALTER COLUMN \"pipeline_id\" SET DEFAULT nextval('ci_pipelines_config_pipeline_id_seq')\n")
   -> 0.0035s
== 20220524102448 DropSequenceTest: reverted (0.0036s) =======================

-CREATE SEQUENCE ci_pipelines_config_pipeline_id_seq
-    START WITH 1
-    INCREMENT BY 1
-    NO MINVALUE
-    NO MAXVALUE
-    CACHE 1;
-
-ALTER SEQUENCE ci_pipelines_config_pipeline_id_seq OWNED BY ci_pipelines_config.pipeline_id;
-
-ALTER TABLE ONLY ci_pipelines_config ALTER COLUMN pipeline_id SET DEFAULT nextval('ci_pipelines_config_pipeline_id_seq'::regclass);

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.