creating down sample modifies the retention duration

[localvar]

Describe the bug（Bug 描述）

I create a database with default retention policy (that's duation is infinite), then I create a down sample with duration set to 70d and found the retention duration of the database becomes 70d too.

as mentioned in #525, I think the usage of duration in down sample is weird.

To Reproduce（Bug 复现步骤）

> create database db0
> use db0
> show retention policies on db0
+---------+----------+--------------------+--------------+---------------+----------------+----------+---------+
|  name   | duration | shardGroupDuration | hot duration | warm duration | index duration | replicaN | default |
+---------+----------+--------------------+--------------+---------------+----------------+----------+---------+
| autogen | 0s       | 168h0m0s           | 0s           | 0s            | 168h0m0s       |        1 | true    |
+---------+----------+--------------------+--------------+---------------+----------------+----------+---------+
8 columns, 1 rows in set

> create DownSample on db0.autogen (float(min,max,count,sum,first,last)) with duration 0s sampleinterval(7d, 14d) timeinterval(1m,3m)
ERR: max sample interval time must be smaller than retention policy duration
> create DownSample on db0.autogen (float(min,max,count,sum,first,last)) with duration 70d sampleinterval(7d, 14d) timeinterval(1m,3m)
> show retention policies on db0
+---------+-----------+--------------------+--------------+---------------+----------------+----------+---------+
|  name   | duration  | shardGroupDuration | hot duration | warm duration | index duration | replicaN | default |
+---------+-----------+--------------------+--------------+---------------+----------------+----------+---------+
| autogen | 1680h0m0s | 168h0m0s           | 0s           | 0s            | 168h0m0s       |        1 | true    |
+---------+-----------+--------------------+--------------+---------------+----------------+----------+---------+
8 columns, 1 rows in set

note the down sample does not allow me to use 0s for infinite duration.

Expected behavior（期望结果）

No response

Screenshots（屏幕截图）

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Logs（完整的错误日志）

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Additional context（其他的一些补充内容）

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