feat: JVM UDF fallback for date_format

[andygrove]

Summary

Adds a JVM UDF fallback so that date_format always runs inside Comet:

• Native (DataFusion to_char) when the format is a literal in the strftime-mappable whitelist and the session timezone is UTC, or when spark.comet.expression.DateFormatClass.allowIncompatible=true.
• JVM UDF (org.apache.comet.udf.DateFormatUDF) in every other case: non-UTC timezone, non-literal format, or a format outside the whitelist. The UDF wraps Spark's own DateFormatClass and is invoked through the existing JvmScalarUdf framework, so results match Spark by construction.

Unlike the regexp engine config, no new user-visible knob is introduced — the JVM UDF is a transparent correctness fallback rather than an opt-in engine.

Behavior change

Cases that previously fell back to Spark (and broke up the Comet pipeline with row-group transitions) now stay inside the Comet operator. The native path is unchanged for the UTC + whitelisted-format case.

Implementation notes

• DateFormatUDF caches one DateFormatClass instance per (format, timezone). Constructing with a literal format makes Spark's formatterOption lazy-val resolve to a reusable formatter, so the per-row work is just eval(InternalRow(micros)).
• For scalar (literal-folded) formats — the common case — the cache lookup is hoisted out of the per-row loop to eliminate Tuple2 + HashMap.get allocations on the hot path.
• The serde now always returns Compatible from getSupportLevel; the native-vs-UDF decision is made inside convert.

Test plan

• CometTemporalExpressionSuite (Spark 3.5): all 27 tests pass
• Three existing fallback-reason tests rewritten to assert the JVM UDF path now runs inside Comet (checkSparkAnswerAndOperator)
• date_format - timestamp_ntz input now runs checkSparkAnswerAndOperator for all timezones (previously only UTC)
• allowIncompatible test fixed: corrected config key (expression, not expr) and switched from answer-comparison to operator-only assertion since the native path may legitimately diverge from Spark for non-UTC timezones
• Spark 3.4 / 4.0 profile sanity
• Benchmark UTC native path vs JVM UDF path on a representative workload

Notes for reviewers

This is a draft pending the items above and any feedback on the routing strategy in CometDateFormat.convert.