Define structure and implement certificate hierarchy to add supported for different certificate types

[lubomirw]

Platform is now supporting only X.509 certificate type. To add support for different certificate types (hybrid, SSH, CV, etc.), we need to define certificate entity hierarchy.

• update DB model if necessary to store certificates of different types
• existing processes, like validation or certificate operations, should be updated to be able to support different certificate types and handle them correctly
• if necessary, update connector interfaces where certificate type needs to be taken into account to perform some operation