feat: adding add_months datetime expression

[athlcode]

Which issue does this PR close?

Part of #4150

Rationale for this change

add_months already has a native implementation in the datafusion-spark crate, but wasn't wired into Comet, so queries using it fell back to Spark.

What changes are included in this PR?

Wires Spark's AddMonths to the datafusion-spark SparkAddMonths UDF using the wire-datafusion-function skill. Also updates the supported-expression docs.

How are these changes tested?

Added a SQL file test (datetime/add_months.sql).

[athlcode]

Hey @coderfender, @comphead. Created this for add_months spark expression, please review, thank you

[coderfender]

Thank you @athlcode , I triggered CI but there seems to be some test failures

[andygrove]

The root cause of the failures is that the upstream expression does not actually have a runnable invoke_with_args. It implements its logic via simplify, which rewrites add_months(d, m) into d + cast(m AS interval year-month) during DataFusion's optimizer pass. Comet's planner builds physical plans straight from the protobuf and does not run the simplifier, so the stub invoke_with_args runs and errors.