Add pluggable cost-based optimizer

[andygrove]

What is the problem the feature request solves?

I would like to add a very simple CBO feature where Comet can call a user-provided class that implements a Comet trait and pass it the original Spark plan and the Comet plan, and have the class determine whether to use the Comet plan or not.

This would allow users to experiment with workload-specific rules to avoid cases where Comet may slow certain queries down.

Describe the potential solution

No response

Additional context

No response