Fixes of service points integration

[AlexisPPLIN]

Happy new year !

I recently started using my service point contribution from v5.1.0, and I realized that everything was broken :face_with_head_bandage: So here is my patch:

Solves

• Sendcloud service points API uses servicepoints.sendcloud.sc URL instead of the standard parcel.sendcloud.sc. So I added a new constant SERVICE_POINTS_BASE_URL.
• Typo in API endpoint service-point -> service-points.
• Made distance optional (null) because only returned by the api if latitude and longitude are provided.

Additions

• How to use the service points API in README.md.
• Test for optional distance value.

Have a nice day :wave:

[villermen]

@AlexisPPLIN I've split the services as per your suggestion and updated the documentation to match. Can you check if I've missed anything? I've already verified this to work on production to be safe.

[AlexisPPLIN]

@villermen I've not tested this in production but LGTM :+1: Obviously this is a breaking change for thoses using ShippingPoint methods (even if it was broken before).