chore: Preparatory work for implementing custom C2R

[andygrove]

Which issue does this PR close?

Closes https://github.com/apache/datafusion-comet/issues/837

Rationale for this change

This is in preparation for implementing an optimized C2R in Comet.

What changes are included in this PR?

• Add CometColumnarToRowExec which is currently an identical copy of Spark's ColumnarToRowExec but with some comments showing how we could use this to specialize for reading data from CometVector
• Adds new optimizer rules to replace ColumnarToRowExec with CometColumnarToRowExec
• Update expected plans

How are these changes tested?

Existing tests