Add a singleton with the last time the stake period work is done

Add a singleton with the last time the stake period work is done

```
/**
* A singleton state object that stores information about stake period calculations.

* This is used to track when the last stake period calculations occurred, so that
* NodeRewards and NodeFee distributions can depend on this singleton instead of tracking it independently.
*/
message StakePeriodInfo {
/**
* The time when the last stake period calculations were done.
*

* This timestamp SHALL be updated at the end of each staking period
* when stake period calculations are processed.
*/
proto.Timestamp last_stake_period_calculation_time = 1;
}
```