Add auto-validation of span.context.destination to MockServer

Agents SHOULD have a generic component used in all tests that validates that the destination information is present for exit spans. Rather than opting into the validation, the testing should provide an opt-out if, for whatever reason, the destination information can't or shouldn't be collected for a particular exit span.

https://github.com/elastic/apm/blob/cb697900cc88fbb4b08e567a77cef68d63f44007/specs/agents/tracing-spans-destination.md