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>

The next session keys for a validator.