feat: add GetTimestamp support via codegen dispatcher

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

Spark's GetTimestamp expression powers the format-based variants of to_timestamp(s, fmt), to_date(s, fmt), to_timestamp_ntz(s, fmt), and try_to_timestamp(s, fmt). Until now Comet had no serde for it, so any plan containing those calls fell back to Spark for the enclosing operator.

A native re-implementation would have to mirror Spark's full SimpleDateFormat / TimestampFormatter parsing semantics, including time parser policy (LEGACY/CORRECTED/EXCEPTION), ANSI failure modes, timezone handling, and locale awareness. The upstream datafusion-spark crate has no equivalent function, so a from-scratch native implementation would be a large project unlikely to be 100% Spark-compatible without significant follow-up work.

The closely-related ToUnixTimestamp already uses the codegen dispatcher (CometCodegenDispatch) to run Spark's own doGenCode inside the Comet pipeline. Wiring GetTimestamp the same way produces bit-exact results today and works uniformly across Spark 3.4, 3.5, and 4.0.

What changes are included in this PR?

• Adds object CometGetTimestamp extends CometCodegenDispatch[GetTimestamp] in spark/src/main/scala/org/apache/comet/serde/datetime.scala.
• Registers classOf[GetTimestamp] -> CometGetTimestamp in the temporalExpressions map in QueryPlanSerde.scala.
• Adds two SQL file tests:
  • spark/src/test/resources/sql-tests/expressions/datetime/get_timestamp.sql covers to_timestamp(s, fmt), to_date(s, fmt), to_timestamp_ntz(s, fmt), try_to_timestamp(s, fmt), NULLs, parse errors, column references, literals, and column-as-format.
  • spark/src/test/resources/sql-tests/expressions/datetime/get_timestamp_ansi.sql covers ANSI mode error throwing with expect_error(CANNOT_PARSE_TIMESTAMP) and the non-throwing try_to_timestamp fallback.

The implement-comet-expression skill was used to scaffold the implementation.

How are these changes tested?

./mvnw test -Pspark-3.5 -Dtest=none -Dsuites="org.apache.comet.CometSqlFileTestSuite get_timestamp" -DwildcardSuites=none

Both new test files pass. The existing to_timestamp_*, try_to_timestamp_*, and related SQL file tests continue to pass under the same profile (12 tests total).