Script to generate elastic migration files

What does this MR do and why?

Adds a script in scripts/ to generate Elastic migration files and introduces a migration dictionary.

The following files are created:

• Elastic migration file ee/elastic/migrate/#{timestamp}_#{file_name}.rb
• Migration spec file ee/spec/elastic/migrate/#{timestamp}_#{file_name}_spec.rb
• Migration dictionary file ee/elastic/docs/#{timestamp}_#{file_name}.yml

Contents of dictionary file:

---
name: CreateEpicIndex
version: '20230704110410'
description: Creates an index for storing and searching epics
group: group::global_search
milestone: '16.2'
introduced_by_url:
obsolete: false
marked_obsolete_by_url:
marked_obsolete_in_milestone:

Where name, description and group as read as generator inputs and timestamp and milestone are based on the current time. The obsolete fields can be used to help support and admins doing upgrades to track when a migration was marked obsolete vs. when it was introduced.

Follow-up MR(s) will backfill dictionary records for all existing migrations.

Contents of migration file:

# frozen_string_literal: true

class CreateEpicIndex < Elastic::Migration
end

Contents of spec file:

# frozen_string_literal: true

require 'spec_helper'
require_relative 'migration_shared_examples'
require File.expand_path('ee/elastic/migrate/20230704110410_create_epic_index.rb')

RSpec.describe CreateEpicIndex, feature_category: :global_search do
  let(:version) { 20230704110410 }
end

How to set up and validate locally

1. Run scripts/elastic-migration and follow the prompts.

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 #414674 (closed)