SDK Eager loading

[alextwoods]

Issue #, if available:

Description of changes:
Adds in eager loading for the SDK to optimize the start up/require times. See: aws/aws-sdk-ruby#3105 for more context and in particular see (revereted) commit that added railties to each gem with eager_load.

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

[mullermp]

Consider a test for eager load (maybe confirm that it's called)

[Schwad]

Yeah maybe something like this

require 'test_helper'

class EagerLoadingTest < ActiveSupport::TestCase
  def test_aws_sdk_rails_eager_loading
    Rails.application.config.eager_load = true

    Aws::Rails.expects(:eager_load!)
    Rails.application.eager_load!
  end
end

Thanks for extracting this!