## What changes were proposed in this pull request?
In the PR, I propose to add new Catalyst type converter for `TimestampType`. It should be able to convert `java.time.Instant` to/from `TimestampType`.
Main motivations for the changes:
- Smoothly support Java 8 time API
- Avoid inconsistency of calendars used inside of Spark 3.0 (Proleptic Gregorian calendar) and `java.sql.Timestamp` (hybrid calendar - Julian + Gregorian).
- Make conversion independent from current system timezone.
By default, Spark converts values of `TimestampType` to `java.sql.Timestamp` instances but the SQL config `spark.sql.catalyst.timestampType` can change the behavior. It accepts two values `Timestamp` (default) and `Instant`. If the former one is set, Spark returns `java.time.Instant` instances for timestamp values.
## How was this patch tested?
Added new testes to `CatalystTypeConvertersSuite` to check conversion of `TimestampType` to/from `java.time.Instant`.
Closes#23811 from MaxGekk/timestamp-instant.
Lead-authored-by: Maxim Gekk <maxim.gekk@databricks.com>
Co-authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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