Allow SQS SendMessage parameters to be specified when using ActiveJob

[cjlarose]

Adds support for specifying SQS SendMessage parameters as described in #646

For example,

class MyJob < ApplicationJob
  def perform(a, b); end
end

MyJob.set(message_group_id: 'abc123',
          message_deduplication_id: 'dedupe',
          message_attributes: {
            'key' => {
              string_value: 'myvalue',
              data_type: 'String'
            }
          })
     .perform_later(123, 'abc')