### What changes were proposed in this pull request?
In the PR, I propose to change `DateTimeUtils.stringToTimestamp` to support any valid time zone id at the end of input string. After the changes, the function accepts zone ids in the formats:
- no zone id. In that case, the function uses the local session time zone from the SQL config `spark.sql.session.timeZone`
- -[h]h:[m]m
- +[h]h:[m]m
- Z
- Short zone id, see https://docs.oracle.com/javase/8/docs/api/java/time/ZoneId.html#SHORT_IDS
- Zone ID starts with 'UTC+', 'UTC-', 'GMT+', 'GMT-', 'UT+' or 'UT-'. The ID is split in two, with a two or three letter prefix and a suffix starting with the sign. The suffix must be in the formats:
- +|-h[h]
- +|-hh[:]mm
- +|-hh:mm:ss
- +|-hhmmss
- Region-based zone IDs in the form `{area}/{city}`, such as `Europe/Paris` or `America/New_York`. The default set of region ids is supplied by the IANA Time Zone Database (TZDB).
### Why are the changes needed?
- To use `stringToTimestamp` as a substitution of removed `stringToTime`, see https://github.com/apache/spark/pull/27710#discussion_r385020173
- Improve UX of Spark SQL by allowing flexible formats of zone ids. Currently, Spark accepts only `Z` and zone offsets that can be inconvenient when a time zone offset is shifted due to daylight saving rules. For instance:
```sql
spark-sql> select cast('2015-03-18T12:03:17.123456 Europe/Moscow' as timestamp);
NULL
```
### Does this PR introduce any user-facing change?
Yes. After the changes, casting strings to timestamps allows time zone id at the end of the strings:
```sql
spark-sql> select cast('2015-03-18T12:03:17.123456 Europe/Moscow' as timestamp);
2015-03-18 12:03:17.123456
```
### How was this patch tested?
- Added new test cases to the `string to timestamp` test in `DateTimeUtilsSuite`.
- Run `CastSuite` and `AnsiCastSuite`.
Closes#27753 from MaxGekk/stringToTimestamp-uni-zoneId.
Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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