DEV: Add converter framework for migrations-tooling (#28540)

* Updates GitHub Actions
* Switches from `bundler/inline` to an optional group in the `Gemfile` because the previous solution didn't work well with rspec
* Adds the converter framework and tests
* Allows loading private converters (see README)
* Switches from multiple CLI tools to a single CLI
* Makes DB connections reusable and adds a new abstraction for the `IntermediateDB`
* `IntermediateDB` acts as an interface for IPC calls when a converter steps runs in parallel (forks). Only the main process writes to the DB.
* Includes a simple example implementation of a converter for now.

migrations/lib/converters/base/parallel_job.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+module Migrations::Converters::Base
+  class ParallelJob
+    def initialize(step)
+      @step = step
+      @stats = ProgressStats.new
+
+      @offline_connection = ::Migrations::Database::OfflineConnection.new
+
+      ::Migrations::ForkManager.after_fork_child do
+        ::Migrations::Database::IntermediateDB.setup(@offline_connection)
+      end
+    end
+
+    def run(item)
+      @stats.reset!
+      @offline_connection.clear!
+
+      begin
+        @step.process_item(item, @stats)
+      rescue StandardError => e
+        @stats.log_error("Failed to process item", exception: e, details: item)
+      end
+
+      [@offline_connection.parametrized_insert_statements, @stats]
+    end
+
+    def cleanup
+    end
+  end
+end