Implementation of create shipment service

With this implementation, we add a service that you can call to create a shipment in Correos and return an object with a label_file that is a PDF file in base64 and the shipment code. Example of usage:

from correos_preregistro.services.shipment import PreRegistrationShipment
from correos_preregistro.client import RawClient
from correos_preregistro.resources import Receiver, Sender, Package
user = "trest_user"
password = "test_password"
client = RawClient(user, password)
receiver = Receiver(
    name="Emilio Jose",
    surname="Marti Gomez",
    address="Cami del corrar, 51, Baix B",
    city="Moralla",
    state="Valencia",
    zip="03015",
    phone="666555444",
    email="emilio.jose@marti.com",
    lang="CA",
)
sender = Sender(
    name="SomConnexio",
    surname="Marti Gomez",
    address="Cami del corrar, 51, Baix B",
    city="Moralla",
    state="Valencia",
    zip="03015",
    phone="666555444",
    email="emilio.jose@marti.com",
    lang="CA",
)
package = Package(
    weight=1,
    postage_type="FP",
    product_code="S0132",
    delivery_modality="ST",
    weight_type="R",
)

shipment = PreRegistrationShipment.create(client=client, code="XXX1", receiver=receiver, sender=sender, package=package)

shipment

shipment.shipment_code
'PQXXX10721396360103015R'
shipment.label_file
'4MDAyMCAwMDAwMCBuIAowMDAwMDgwMjkyIDAwMDAwIG4gCjAwMDAwODA1NTQgMDAwMDAgbi .... 4MDAyMCAwMDAwMCBuIAowMDAwMDgwMjkyIDAwMDAwIG4gCjAwMDAwODA1NTQgMDAwMDAgbi'

TODO

• Fix usage of etree or Zeep libraries.
• Add tests (services and resources).
• Logging??