Implement mock implementation for subscriptions

[okkevandereijk]

Currently there's no real mock implementation for subscriptions. This occurs issues like labd/terraform-provider-commercetools#364 (comment) appear in our Terraform provider, as we don't see any error appear if we miss certain properties. We need to ensure we have a proper mock implementation to overcome this.

[korsvanloon]

There is a subscription repository, service and generated draft schema now, so the resource itself is mocked. #426 fixes the part that matches the complaint in this issue: the generated DestinationSchema was only { type: string } (the generator flattens the Destination union to its base type), so a draft with an incomplete destination — an SQS destination without a region — was accepted even in strict mode. That PR validates the destination as a discriminated union over all seven destination types, and fills in the missing update actions (changeDestination, setChanges, setEvents, setMessages).

Two things worth flagging for whoever picks up the rest:

• Draft validation only runs with strict: true (AbstractService.post checks repository.strict), so the Terraform provider needs that option for any of this validation to fire.
• The destination is never actually contacted. commercetools sends a test message when a subscription is created and refuses the subscription if it cannot be delivered; the mock fakes that with a hardcoded 0000000000 AWS account id in SubscriptionRepository.create. Doing this properly means deciding how far a test double should go — whether to attempt real delivery, expose a hook so a test can make delivery fail, or keep faking it.

Labelling help wanted for that last part, since it needs a decision rather than an implementation.