Add native Flink SQL timestamp and hash functions

[yangzhg]

Fixes #638

Summary

• Add native Flink SQL implementations for TIMESTAMPDIFF, TO_TIMESTAMP, and HASH_CODE.
• Register Flink SQL signatures for the supported Bolt type surface.
• Add coverage for Flink-compatible month differences, timestamp parsing fallback, and Java-style hash code results.

Flink compatibility notes

• TIMESTAMPDIFF follows Flink/Calcite month boundary behavior through SqlFunctionUtils.subtractMonths.
• TO_TIMESTAMP keeps Flink's default format selection and fallback to Timestamp.valueOf / Date.valueOf for non-zero-padded dates.
• HASH_CODE follows Flink codegen behavior from CodeGenUtils.hashCodeForType.

[Copilot]

Pull request overview

This PR adds native Flink SQL implementations for key datetime and hashing functions to improve Flink-compatibility of Bolt’s FlinkSQL function surface (Fixes #638).

Changes:

• Introduces Flink-native TIMESTAMPDIFF with Calcite/Flink month-boundary semantics and Java-style int narrowing behavior.
• Implements TO_TIMESTAMP with Flink-like format selection, plus a parsing fallback for non-zero-padded dates/timestamps.
• Adds HASH_CODE for common scalar types plus DECIMAL support via a vector function, and expands FlinkSQL test coverage accordingly.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
bolt/functions/prestosql/DateTimeImpl.h	Switches Presto datetime diff helpers to a shared lib header (TimeDiffUtils.h).
bolt/functions/lib/TimeDiffUtils.h	New shared diffTimestamp / diffDate helpers extracted for reuse.
bolt/functions/flinksql/ToTimestampFunction.h	New Flink to_timestamp implementation with auto-format selection and fallback parsing.
bolt/functions/flinksql/tests/FlinkSqlDataTimeFunctionsTest.cpp	Adds extensive coverage for timestampdiff, to_timestamp, and hash_code Flink-compatible behavior.
bolt/functions/flinksql/registration/Register.cpp	Registers new FlinkSQL functions (timestampdiff, to_timestamp, hash_code) across supported types.
bolt/functions/flinksql/HashCodeFunction.h	New Flink hash_code scalar implementation for supported primitive/string/timestamp types.
bolt/functions/flinksql/HashCodeFunction.cpp	Adds DECIMAL hash_code vector function + signatures for DECIMAL support.
bolt/functions/flinksql/DateTimeDiff.h	New Flink timestampdiff implementation with Calcite/Flink month subtraction semantics.
bolt/functions/flinksql/CMakeLists.txt	Adds HashCodeFunction.cpp to the Flink function implementation library build.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[luozenglin]

LGTM