Prevent unnecessary jsonb updates for builds_metadata

What does this MR do and why?

While investigating gitlab-com/gl-infra/production#7907 (closed) we've noticed that the requests for job assignment to /api/:version/jobs/request were updating the config_options column on ci_builds_metadata table:

Process 3091143: /*application:web,correlation_id:eb6a2d49457ed84be0c62fb87dbf97cf,endpoint_id:POST /api/:version/jobs/request,db_config_name:ci*/ UPDATE "ci_builds_metadata" SET "config_options" = '[REDACTED]' WHERE "ci_builds_metadata"."id" = 3028893490

This should not happen because we don't have any business logic to change the values stored in that column after a pipeline is created.

This is started by transitioning the job into the running state which triggers a transition callback:

      after_transition pending: :running do |build|
        build.ensure_metadata.update_timeout_state
      end

This callback calls update on the Ci::BuildMetadata record:

    def update_timeout_state
      timeout = timeout_with_highest_precedence

      return unless timeout

      update(timeout: timeout.value, timeout_source: timeout.source)
    end

Accessing the record would trigger config_options to be registered as changed:

[1] pry(main)> m.assign_attributes(timeout: 120, timeout_source: 1)
=> nil
[2] pry(main)> m.changes
=> {"timeout"=>[nil, 120],
 "config_options"=>
  [{"image"=>{"name"=>"busybox:latest"},
    "script"=>["echo \"Do another parallel test here\"", "echo \"For example run a lint test\""],
    "after_script"=>["echo \"After script section\"", "echo \"For example you might do some cleanup here\""],
    "before_script"=>["echo \"Before script section\"", "echo \"For example you might run an update here or install a build dependency\"", "echo \"Or perhaps you might print out some debugging details\""]},
   {"image"=>{"name"=>"busybox:latest"},
    "script"=>["echo \"Do another parallel test here\"", "echo \"For example run a lint test\""],
    "after_script"=>["echo \"After script section\"", "echo \"For example you might do some cleanup here\""],
    "before_script"=>["echo \"Before script section\"", "echo \"For example you might run an update here or install a build dependency\"", "echo \"Or perhaps you might print out some debugging details\""]}]}
[3] pry(main)> m.changes["config_options"].reduce(:==)
=> true

If the timeout and timeout_source values do not change, we end up updating only the serialized JSONb values.

Screenshots or screen recordings

Before

[1] pry(main)> metadata = Ci::BuildMetadata.last; nil
  Ci::BuildMetadata Load (0.9ms)  SELECT "ci_builds_metadata".* FROM "ci_builds_metadata" ORDER BY "ci_builds_metadata"."id" DESC LIMIT 1 /*application:console,db_config_name:ci,line:(pry):1:in `__pry__'*/
=> nil
[2] pry(main)> metadata.changes
=> {}
[3] pry(main)> metadata.config_options; nil
=> nil
[4] pry(main)> metadata.changes.keys
=> ["config_options"]

After

[2] pry(main)> metadata = Ci::BuildMetadata.last; nil
  Ci::BuildMetadata Load (0.7ms)  SELECT "ci_builds_metadata".* FROM "ci_builds_metadata" ORDER BY "ci_builds_metadata"."id" DESC LIMIT 1 /*application:console,db_config_name:ci,line:(pry):2:in `__pry__'*/
=> nil
[3] pry(main)> metadata.changes
=> {}
[4] pry(main)> metadata.config_options; nil
=> nil
[5] pry(main)> metadata.changes.keys
=> []

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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