feat(agents): support persistense feature for planner agents

[antoniibelyshev]

This PR extends the persistense feature to be available for the planner agents.

Example:

val agent = AIAgent(
    promptExecutor = promptExecutor,
    strategy = AIAgentPlannerStrategy("my-strategy", planner),
    agentConfig = agentConfig
) {
    install(Persistence) {
        this.storage = provider
    }
}

note: data class AgentCheckpointData was updated: fields nodePath, lastInput, and lastOutput were removed (they are now stored in the properties: JSONObject which was there before). Since the intended use-case for this class from the user perspective is to only serialize-deserialize it, the users should not be affected by that, therefore I do not mark this as a braking change.

closes KG-673