add middleware to validate/insert request ids

[CrowdHailer]

Middleware should check that the x-request-id header is set. If there is no value and a generate has been given it will insert missing request id.

use Raxx.RequestID, generate: :generate_request_id, validate: validate_request_id

def generate(_config) do
  UUID.uuid4()
end

def validate(request_id) do
  {:ok, request_id}
end

If no validate is given then any request id should be assumed valid.

See https://github.com/elixir-plug/plug/blob/master/lib/plug/request_id.ex

[CrowdHailer]

Done in #101