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/scripts/generate_schema

@@ -11,7 +11,6 @@ require_relative "../lib/migrations"
 
 module Migrations
   load_rails_environment
-  load_gemfiles("common")
 
   class SchemaGenerator
     def initialize(opts = {})
@@ -305,4 +304,4 @@ module Migrations
   end
 end
 
-Migrations::SchemaGenerator.new(output_file_path: ARGV.first).run
+::Migrations::SchemaGenerator.new(output_file_path: ARGV.first).run