feat(datetime): prototype JVM UDF path for Hour/Minute/Second (engine=java)

[andygrove]

Which issue does this PR close?

Part of #4311.

Rationale for this change

Comet's native Rust implementations of hour, minute, and second incorrectly apply timezone conversion to TimestampNTZType inputs (#3180). Today this means Comet falls back to Spark for the NTZ case, losing native execution for the rest of the stage.

This PR prototypes the approach outlined in #4311: add an opt-in JVM UDF path that produces bit-exact Spark results for the affected expressions, in exchange for a JNI round-trip per native batch. It mirrors the engine-selection idiom proposed in the open regex PR (#4239), keeping the framing consistent across families. The default stays rust; users opt into java for full Spark compatibility.

The scope is intentionally narrow: three closely related expressions sharing a serde shape and a UDF base class. Once this engine-config model is reviewed, it extends naturally to TruncTimestamp, DateFormat, and the field-extraction expressions in a follow-up.

What changes are included in this PR?

• New config spark.comet.exec.datetime.engine with values rust (default) and java. Lives in CometConf.scala.
• New DateTimeFieldUDF abstract base in common/src/main/scala/org/apache/comet/udf/, implementing the Arrow vector iteration, null preservation, and TZ/NTZ branching using java.time. Concrete subclasses HourUDF, MinuteUDF, SecondUDF each supply a one-line extract(LocalDateTime): Int.
• Fork in CometHour/CometMinute/CometSecond (in spark/src/main/scala/org/apache/comet/serde/datetime.scala): when engine=java, the serde builds a JvmScalarUdf proto pointing at the matching UDF class; when engine=rust, the existing native path is unchanged. A shared private DateTimeFieldUdfHelper.buildProto factors the proto construction.
• The engine=java branch in getSupportLevel matches explicitly on TimestampType | TimestampNTZType and returns Unsupported for anything else, so a future input-type change in Spark falls back cleanly instead of crashing the UDF at runtime.
• Compat doc: new "Engine Selection (experimental)" section in docs/source/user-guide/latest/compatibility/expressions/datetime.md.

No native Rust changes; the JvmScalarUdf proto and JvmScalarUdfExpr planner integration were already merged in #4232.

This PR was scaffolded with the project's brainstorming and writing-plans skills.

How are these changes tested?

Three test suites, totaling 17 new tests:

• DateTimeFieldUDFSuite (Scala unit, in the spark module): 8 tests exercising each UDF directly on Arrow vectors. Covers TZ + NTZ, multiple session timezones, and null preservation.
• CometDatetimeJvmSuite (Spark integration): 8 tests with engine=java set in sparkConf. Each of hour/minute/second is checked across UTC, America/Los_Angeles, and Asia/Tokyo on both TimestampType and TimestampNTZ, plus a DST fall-back boundary test in LA and a plan-inspection test asserting native execution. All use checkSparkAnswerAndOperator to verify both correctness against Spark and that the operator ran natively.
• CometDatetimeEngineDefaultSuite (Spark integration): 1 test asserting the config defaults to rust and that the rust path still runs natively under defaults. Regression guard for the existing path.

Also verified locally: spotless:apply is a no-op, cargo clippy --all-targets --workspace -- -D warnings is clean, and CometTemporalExpressionSuite (27 tests) still passes unchanged, confirming the rust default path is untouched.