Support micro seconds timestamp precision with copy unload

[arthurli1126]

Hi folks, due to high NAT gateway cost, we have to use copy load when reading from snowflake, but currently copy unload doesn't support micro second level precision(only at mills). I can work on the PR to add it. But wondering if you have any concerns about this.

[sfc-gh-mrui]

@arthurli1126 Could you please try SC 2.6.0 or newer version? COPY UNLOAD is mainly used from SC 2.5.x and prior versions.

[arthurli1126]

@sfc-gh-mrui thanks for your reply, the problem is with the simpleDateTime(https://github.com/snowflakedb/spark-snowflake/blob/master/src/main/scala/net/snowflake/spark/snowflake/Conversions.scala#L66) used for parse timestamp during copy unload. The format only support millisecond precision and would give wrong timestamp if the timestamp carries micro seconds. For instance, for string 2023-03-01 07:54:56.191173 it would consider it carries 191173 milliseconds so it will add
191000 / 1000 / 60 = 3 mins 11s and put 173 microseconds to milliseconds filed:
2023-03-01 07:58:07.173000.

[arthurli1126]

Created a draft PR for this: #492

Please let me know if I misunderstand anything or if you have any concerns about the approach.