### What changes were proposed in this pull request?
This PR is to support parsing timestamp values with variable length second fraction parts.
e.g. 'yyyy-MM-dd HH:mm:ss.SSSSSS[zzz]' can parse timestamp with 0~6 digit-length second fraction but fail >=7
```sql
select to_timestamp(v, 'yyyy-MM-dd HH:mm:ss.SSSSSS[zzz]') from values
('2019-10-06 10:11:12.'),
('2019-10-06 10:11:12.0'),
('2019-10-06 10:11:12.1'),
('2019-10-06 10:11:12.12'),
('2019-10-06 10:11:12.123UTC'),
('2019-10-06 10:11:12.1234'),
('2019-10-06 10:11:12.12345CST'),
('2019-10-06 10:11:12.123456PST') t(v)
2019-10-06 03:11:12.123
2019-10-06 08:11:12.12345
2019-10-06 10:11:12
2019-10-06 10:11:12
2019-10-06 10:11:12.1
2019-10-06 10:11:12.12
2019-10-06 10:11:12.1234
2019-10-06 10:11:12.123456
select to_timestamp('2019-10-06 10:11:12.1234567PST', 'yyyy-MM-dd HH:mm:ss.SSSSSS[zzz]')
NULL
```
Since 3.0, we use java 8 time API to parse and format timestamp values. when we create the `DateTimeFormatter`, we use `appendPattern` to create the build first, where the 'S..S' part will be parsed to a fixed-length(= `'S..S'.length`). This fits the formatting part but too strict for the parsing part because the trailing zeros are very likely to be truncated.
### Why are the changes needed?
improve timestamp parsing and more compatible with 2.4.x
### Does this PR introduce any user-facing change?
no, the related changes are newly added
### How was this patch tested?
add uts
Closes#27906 from yaooqinn/SPARK-31150.
Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
0946a9514f