feature: `MonthMod` shard algorithm

[dk-lockdown]

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implement ShardingAlgorithm interface.

type ShardingAlgorithm interface {
	HasShardingKey(key string) bool
	Shard(condition *KeyCondition) (Condition, error)
	ShardRange(cond1, cond2 *KeyCondition) (Condition, error)
	AllShards() Condition
	AllowFullScan() bool
}

https://github.com/CECTC/dbpack/blob/0ae8c774106b58f5dbbfa045d4a8591fb913c929/pkg/cond/sharding_algorithm.go#L226

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[tanryberdi]

@dk-lockdown could you please explain with more details? 🙏 I was not clear for me, please man

[dk-lockdown]

@dk-lockdown could you please explain with more details? 🙏 I was not clear for me, please man

If a column of the table is of type date, and the sharding algorithm is sharded by month. When execute select * from t where day > 2020-01-05 and day < 2020-05-05, and the sharding key is day, the sharding algorithm is Month(day) mod 12, the sharding result will be calculated for January to May, so the sql will be rewritten as:

select * from t_1 where day > 2020-01-05 and day < 2020-05-05;
select * from t_2 where day > 2020-01-05 and day < 2020-05-05;
select * from t_3 where day > 2020-01-05 and day < 2020-05-05;
select * from t_4 where day > 2020-01-05 and day < 2020-05-05;
select * from t_5 where day > 2020-01-05 and day < 2020-05-05;

Then aggregate the query results.