Consolidate the loading of modules related to AWS SQS ActiveJob

[kakubin]

partially related: #133

We are using the gem just for processing async job on ActiveJob.
Since we don't need any other functionality of the gem, we would like a way to use it easily like the following.

gem 'aws-sdk-rails', require: false

require 'aws/rails/sqs_active_job' # load modules just about worker

module MyApplication
  class Application < Rails::Application
    config.active_job.queue_adapter = :sqs
  end
end

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license.

[mullermp]

Thanks. This seems fine. Would you be able to also move the associated class config tests out of the configuration spec and into a new spec file that matches where they exist now?

[kakubin]

Would you be able to also move the associated class config tests out of the configuration spec and into a new spec file that matches where they exist now?

Done

[kakubin]

To be honest, since other dependencies like aws-sdk-s3 are not needed in my case, I would like AWS SQS ActiveJob to be separated into a gem and managed similarly to aws-sdk-ruby.

[mullermp]

Hm. I wonder if we can drop direct dependency and require them to be loaded instead. I'll think about that.

[mullermp]

Thanks for your fix. I'm open to any PRs for making other features optional and not having other SDK gems as direct dependencies.