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/cli/import_command.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Migrations::CLI::ImportCommand
+  def self.included(thor)
+    thor.class_eval do
+      desc "import", "Import a file"
+      def import
+        require "extralite"
+
+        puts "Importing into Discourse #{Discourse::VERSION::STRING}"
+        puts "Extralite SQLite version: #{Extralite.sqlite3_version}"
+      end
+    end
+  end
+end