## What changes were proposed in this pull request?
Changed type of `sec` argument in the `make_timestamp()` function from `DOUBLE` to `DECIMAL(8, 6)`. The scale is set to 6 to cover microsecond fractions, and the precision is 2 digits for seconds + 6 digits for microsecond fraction. New type prevents losing precision in some cases, for example:
Before:
```sql
spark-sql> select make_timestamp(2019, 8, 12, 0, 0, 58.000001);
2019-08-12 00:00:58
```
After:
```sql
spark-sql> select make_timestamp(2019, 8, 12, 0, 0, 58.000001);
2019-08-12 00:00:58.000001
```
Also switching to `DECIMAL` fixes rounding `sec` towards "nearest neighbor" unless both neighbors are equidistant, in which case round up. For example:
Before:
```sql
spark-sql> select make_timestamp(2019, 8, 12, 0, 0, 0.1234567);
2019-08-12 00:00:00.123456
```
After:
```sql
spark-sql> select make_timestamp(2019, 8, 12, 0, 0, 0.1234567);
2019-08-12 00:00:00.123457
```
## How was this patch tested?
This was tested by `DateExpressionsSuite` and `pgSQL/timestamp.sql`.
Closes#25421 from MaxGekk/make_timestamp-decimal.
Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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