Refactor creating Kea commands

The Kea commands are created in several places in Stork. We construct them by writing the command name freely and passing the raw dictionary as arguments. It is error-prone and duplicates the code.

We should refactor it. We can define a dedicated constructor for each command to accept the necessary arguments in a structured form and ensure the command name is correct.

Additionally, the constructor could return the expected response type. Currently, the response definitions are distributed over the application, or we use raw maps to access data.