Add support for `explode_outer`

[andygrove]

What is the problem the feature request solves?

#2836 adds explode_outer but it is marked as incompatible due to UnnestExec having different semantics to Spark for empty arrays. See apache/datafusion#19053 for more information.

This issue is to track implementing the correct behavior in Comet so that this expression can be enabled by default.

There are existing tests in CometGenerateExecSuite, which are currently ignored.

We need to add a microbenchmark as well.

Describe the potential solution

No response

Additional context

No response