Reference the API XSD Schema

[judgej]

There is a schema that gives us most of what we need to know about the objects:

https://api.authorize.net/xml/v1/schema/AnetApiSchema.xsd

It does not cover webhooks. If we are not auto-generating code from this schema, then we could at least use it to generate validation rules or tests.

The name "authorizenet-schema" feels like it would have been a better name than "authorizenet-objects" too.

[judgej]

The schema is in many cases more up-to-date, more correct and more complete than the API reference documentation. They should be used together.

[judgej]

There may not be a schema for version 2.0 of the API, which is what we really need.