ping-check-hook-task-3 Implement PingContext and PingContextStore
Implement PingContext and PingContextStore classes per Ping Check design
https://gitlab.isc.org/isc-projects/kea/-/wikis/Designs/ping-check
When complete: PingContextStore should provide CRUD operations for PingContexts, as well as requisite finder methods. This implies that PingContext is fully functional (ctor, accessors, state etc...)