DEV: Prepare new structure for migrations-tooling (#26631)

* Moves existing files around. All essential scripts are in `migrations/bin`, and non-essential scripts like benchmarks are in `migrations/scripts`
* Dependabot configuration for migrations-tooling (disabled for now)
* Updates test configuration for migrations-tooling
* Shorter configuration for intermediate DB for now. We will add the rest table by table.
* Adds a couple of benchmark scripts
* RSpec setup especially for migrations-tooling and the first tests
* Adds sorting/formatting to the `generate_schema` script

migrations/lib/migrations.rb

@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+require "bundler/inline"
+require "bundler/ui"
+
+module Migrations
+  def self.root_path
+    @root_path ||= File.expand_path("..", __dir__)
+  end
+
+  def self.load_gemfiles(*relative_paths)
+    gemfiles_root_path = File.join(Migrations.root_path, "config/gemfiles")
+
+    relative_paths.each do |relative_path|
+      path = File.join(File.expand_path(relative_path, gemfiles_root_path), "Gemfile")
+
+      unless File.exist?(path)
+        warn "Could not find Gemfile at #{path}"
+        exit 1
+      end
+
+      gemfile_content = File.read(path)
+
+      # Create new UI and set level to confirm to avoid printing unnecessary messages
+      bundler_ui = Bundler::UI::Shell.new
+      bundler_ui.level = "confirm"
+
+      begin
+        gemfile(true, ui: bundler_ui) do
+          # rubocop:disable Security/Eval
+          eval(gemfile_content, nil, path, 1)
+          # rubocop:enable Security/Eval
+        end
+      rescue Bundler::BundlerError => e
+        warn "\e[31m#{e.message}\e[0m"
+        exit 1
+      end
+    end
+  end
+
+  def self.load_rails_environment(quiet: false)
+    puts "Loading application..." unless quiet
+
+    rails_root = File.expand_path("../..", __dir__)
+    # rubocop:disable Discourse/NoChdir
+    Dir.chdir(rails_root) { require File.join(rails_root, "config/environment") }
+    # rubocop:enable Discourse/NoChdir
+  end
+
+  def self.configure_zeitwerk(*directories)
+    require "zeitwerk"
+
+    root_path = Migrations.root_path
+
+    loader = Zeitwerk::Loader.new
+    directories.each do |dir|
+      loader.push_dir(File.expand_path(dir, root_path), namespace: Migrations)
+    end
+    loader.setup
+  end
+end