Reuse parquet-io-java date/time conversion helpers

[kaklakariada]

Review follow-up from PR #394 for issue #393.

Original PR: #394
Original review comment: #394 (comment)

The local DateTimeConverter overlaps strongly with parquet-io-java's com.exasol.parquetio.reader.converter.DateTimeHelper from parquet-io-java 2.0.16. Both provide conversions for timestamps from micros/millis, micros from timestamps, micros from Julian day, days since epoch, and date from epoch days.

Proposed direction:

• Check whether parquet-io-java can expose a public date/time conversion API suitable for this project.
• Replace the duplicated local conversion code with that API where possible.
• Keep behavior compatible with the existing tests and supported timestamp/date edge cases.