Add RequestId.with_request_id.

I'd like to change our webhooks to keep track of the original request_id (the one kicked off by the user), so I'd like to do this:

```ruby
desc 'Update the status of a delivery'
params do
  optional :request_id,
    type: String,
    desc: 'The original request_id.',
    default: RequestId.request_id
end
post '/:id/status_callback' do
  authenticate!

  RequestId.with_request_id params.request_id do
    AsyncMethod.perform_async( ... ) if message_status
  end

  status 200
end

```