NextKeys: StorageDescriptor<
    [Key: SS58String],
    {
        authority_discovery: FixedSizeBinary<32>;
        babe: FixedSizeBinary<32>;
        beefy: FixedSizeBinary<33>;
        grandpa: FixedSizeBinary<32>;
        para_assignment: FixedSizeBinary<32>;
        para_validator: FixedSizeBinary<32>;
    },
    true,
    never,
>

The next session keys for a validator.