Elastic migration helper for creating a new index

What does this MR do and why?

Adds an Elastic migration helper for creating a new index: Elastic::MigrationCreateIndex that can be called as follows:

class MigrationName < Elastic::Migration
  include Elastic::MigrationCreateIndex

  retry_on_failure

  def document_type
    :epic
  end

  def target_class
    Epic
  end
end

This will be used to create the Epic index.

How to set up and validate locally

Note: in order to test, you will need to delete an existing index (and loose the data).

1. Delete the user index: Gitlab::Elastic::Helper.default.delete_index(index_name: 'gitlab-development-users')
2. Gitlab::Elastic::Helper.default.index_exists?(index_name: 'gitlab-development-users') => false
3. Create a new migration: touch ee/elastic/migrate/20230531135700_create_temp_user_index
4. Add the following to the file:

   class CreateTempUserIndex < Elastic::Migration
     include Elastic::MigrationCreateIndex
   
     retry_on_failure
   
     def document_type
       :user
     end
   
     def target_class
       User
     end
   end

5. Execute the migration worker a few times: Elastic::MigrationWorker.new.perform
6. Gitlab::Elastic::Helper.default.index_exists?(index_name: 'gitlab-development-users') => true
7. Delete the temporary migration that was created.
8. Reindex users: bundle exec rake gitlab:elastic:index_users

Example logs

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.

Related to #250699 (closed)