feat: support elt expression

[n0r0shi]

Summary

• Wire elt from the datafusion-spark crate (SparkElt) to Comet
• Register in jni_api.rs and add serde mapping via CometScalarFunction in QueryPlanSerde.scala

Test plan

• New test in CometStringExpressionSuite — passes
• Spotless formatting check passes
• Rust native build passes

[andygrove]

Serde wiring and UDF registration look good. Same clean pattern as your other PRs.

A couple of test suggestions. The SQL file framework in spark/src/test/resources/sql-tests/expressions/string/ would be a nice fit here instead of CometStringExpressionSuite.

The Spark reference mentions that elt returns null for out-of-range indices in non-ANSI mode and throws an error in ANSI mode. It might be worth adding a test for out-of-range index values like elt(0, 'a', 'b') or elt(5, 'a', 'b', 'c') to make sure those match Spark's behavior.