### What changes were proposed in this pull request?
Added new expression `TimestampDiff` for timestamp subtractions. It accepts 2 timestamp expressions and returns another one of the `CalendarIntervalType`. While creating an instance of `CalendarInterval`, it initializes only the microsecond field by difference of the given timestamps in microseconds, and set the `months` field to zero. Also I added an rule for conversion `Subtract` to `TimestampDiff`, and enabled already ported test queries in `postgreSQL/timestamp.sql`.
### Why are the changes needed?
To maintain feature parity with PostgreSQL which allows to get timestamp difference:
```sql
# select timestamp'today' - timestamp'yesterday';
?column?
----------
1 day
(1 row)
```
### Does this PR introduce any user-facing change?
Yes, previously users got the following error from timestamp subtraction:
```sql
spark-sql> select timestamp'today' - timestamp'yesterday';
Error in query: cannot resolve '(TIMESTAMP('2019-10-04 00:00:00') - TIMESTAMP('2019-10-03 00:00:00'))' due to data type mismatch: '(TIMESTAMP('2019-10-04 00:00:00') - TIMESTAMP('2019-10-03 00:00:00'))' requires (numeric or interval) type, not timestamp; line 1 pos 7;
'Project [unresolvedalias((1570136400000000 - 1570050000000000), None)]
+- OneRowRelation
```
after the changes they should get an interval:
```sql
spark-sql> select timestamp'today' - timestamp'yesterday';
interval 1 days
```
### How was this patch tested?
- Added tests for `TimestampDiff` to `DateExpressionsSuite`
- By new test in `TypeCoercionSuite`.
- Enabled tests in `postgreSQL/timestamp.sql`.
Closes#26022 from MaxGekk/timestamp-diff.
Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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