Implement validation service for billing documents

[onthebreeze]

In general, the validation service implementation for each semantic spec will be in the corresponding testpoint repo and invoked like this

https://bill.testpoint.io/api/v0/validator

The API spec should comply with https://github.com/ausdigital/ausdigital-json/blob/master/docs/ValidationAPI.md

And the validation rules for the billing document validator should comply with https://github.com/ausdigital/ausdigital-bill/blob/master/docs/Billing-Documents.md