## What changes were proposed in this pull request?
I propose new levels of truncations for the `date_trunc()` and `trunc()` functions:
1. `MICROSECOND` and `MILLISECOND` truncate values of the `TIMESTAMP` type to microsecond and millisecond precision.
2. `DECADE`, `CENTURY` and `MILLENNIUM` truncate dates/timestamps to lowest date of current decade/century/millennium.
Also the `WEEK` and `QUARTER` levels have been supported by the `trunc()` function.
The function is implemented similarly to `date_trunc` in PostgreSQL: https://www.postgresql.org/docs/11/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC to maintain feature parity with it.
Here are examples of `TRUNC`:
```sql
spark-sql> SELECT TRUNC('2015-10-27', 'DECADE');
2010-01-01
spark-sql> set spark.sql.datetime.java8API.enabled=true;
spark.sql.datetime.java8API.enabled true
spark-sql> SELECT TRUNC('1999-10-27', 'millennium');
1001-01-01
```
Examples of `DATE_TRUNC`:
```sql
spark-sql> SELECT DATE_TRUNC('CENTURY', '2015-03-05T09:32:05.123456');
2001-01-01T00:00:00Z
```
## How was this patch tested?
Added new tests to `DateTimeUtilsSuite`, `DateExpressionsSuite` and `DateFunctionsSuite`, and uncommented existing tests in `pgSQL/date.sql`.
Closes#25336 from MaxGekk/date_truct-ext.
Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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