SubPoolsStorage: StorageDescriptor<
    [Key: number],
    {
        no_era: { balance: bigint; points: bigint };
        with_era: [number, { balance: bigint; points: bigint }][];
    },
    true,
    never,
>

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.