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/spec/rails_helper.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+# we need to require the rails_helper from core to load the Rails environment
+require_relative "../../spec/rails_helper"
+
+require "bundler/inline"
+require "bundler/ui"
+
+# this is a hack to allow us to load Gemfiles for converters
+Dir[File.expand_path("../config/gemfiles/**/Gemfile", __dir__)].each do |path|
+  # Create new UI and set level to confirm to avoid printing unnecessary messages
+  bundler_ui = Bundler::UI::Shell.new
+  bundler_ui.level = "confirm"
+
+  gemfile(true, ui: bundler_ui) do
+    # rubocop:disable Security/Eval
+    eval(File.read(path), nil, path, 1)
+    # rubocop:enable Security/Eval
+  end
+end