Sessions: StorageDescriptor<
    [Key: number],
    {
        active_validator_indices: number[];
        assignment_keys: FixedSizeBinary<32>[];
        discovery_keys: FixedSizeBinary<32>[];
        dispute_period: number;
        n_cores: number;
        n_delay_tranches: number;
        needed_approvals: number;
        no_show_slots: number;
        random_seed: FixedSizeBinary<32>;
        relay_vrf_modulo_samples: number;
        validator_groups: number[][];
        validators: FixedSizeBinary<32>[];
        zeroth_delay_tranche_width: number;
    },
    true,
    never,
>

Session information in a rolling window. Should have an entry in range EarliestStoredSession..=CurrentSessionIndex. Does not have any entries before the session index in the first session change notification.