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/benchmarks/write.rb

@@ -5,7 +5,7 @@ require "bundler/inline"
 
 gemfile(true) do
   source "https://rubygems.org"
-  gem "extralite-bundle", github: "digital-fabric/extralite"
+  gem "extralite-bundle"
 end
 
 require "etc"
@@ -43,7 +43,7 @@ def with_db_path
   yield tempfile.path
 
   db = create_extralite_db(tempfile.path)
-  row_count = db.query_single_value("SELECT COUNT(*) FROM users")
+  row_count = db.query_single_splat("SELECT COUNT(*) FROM users")
   puts "Row count: #{row_count}" if row_count != ROW_COUNT
   db.close
 ensure