Add new Kea config checkers

We have gathered the following ideas for the new config checkers:

• 1. if there is only one subnet in a shared network, suggest disabling shared network;
• 2. if there is in-pool reservation enabled, but there are no in-pool reservations, suggest out-of-pool as better performant;
• 3. if there is custom option definition, but no option-data that uses it, suggest removing unused defintions;
• 4. if there are subnets without any pools and no reservations, suggest removing unused subnets;
• 5. inspect HA configs of both servers and make sure there are no discrepancies;
• 6. it's possible to misconfigure ports in HA+MT configuration, so it's still connecting via CA rather than with DHCP directly. #611
• 7. if there is the Stork Agent configured to use Basic Auth, but the Kea CAs use HTTP only (@slawek)
• 8. you might check whether the customer has both the subnet commands hook and the config backend hook. a lot of people don't seem to understand these are sort of mutually exclusive (@vicky) #940 (closed)
• 9. db backends configured and no secrets provided for the connection (@vicky)
• 10. if your pool equals the number of reservations, warn that devices not in the reserved list will not get an address (support) #941 (closed)
• 11. HA ST but Kea MT - suggest to enable MT for HA #611

We could pick one, two or three and add them for the 1.0. Other to be postponed.

UPDATE: A nice list of ideas was discussed on DHCP/support call on 2021-11-03.