Add domain validation API

[Integralist]

This PR implements the following API endpoints:

• https://developer.fastly.com/reference/api/services/domain/#check-domains
• https://developer.fastly.com/reference/api/services/domain/#check-domain

[Integralist]

@kailan @doramatadora @triblondon

This is an interesting example of where code generating the go-fastly client might present problems.

e.g. it's unlikely that the generated code will try to handle this situation (i.e. multiple types within an array) intelligently as it won't know the context of the response data.

I imagine it will more likely just generate code that uses the empty interface type interface{}, which will cause more problems for consumers of the library as they'd then need to type assert the interface{} into a concrete type.