SubPoolsStorage: StorageDescriptor<[Key: number], {
    no_era: {
        balance: bigint;
        points: bigint;
    };
    with_era: [number, {
        balance: bigint;
        points: bigint;
    }];
}, true>

Groups of unbonding pools. Each group of unbonding pools belongs to a bonded pool, hence the name sub-pools. Keyed by the bonded pools account.