Implement the control point action "Eval: Evaluate an expression".

This might have limited usability since it cannot have any action configuration variables, or action state variables.

The problem is how to pass an arbitrary C expression which can contain commas as a macro argument to the macro that defines a control point with this action. It might be easy, hard, or impossible. I don't know which. That is why it is in Future. If that is easy, this could be moved to MVP.

An alternative to this is control point action "NOP: No operation" in WT-13571. However "Eval" is easier to use and less conspicuous.

Note, the control points implementation tickets are in three time periods:

1. POC - Part of the POC.
2. MVP - Part of the minimal viable product. However, even the POC can be used but less conveniently than the MVP.
3. Future - Enhancements after the MVP.

The title of each ticket starts with the time period.