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>

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.