[ENH] Readonly Workflow Variables

## Overview
We need the ability to mark workflow variables as readonly. This feature will ensure that once a variable is set, it cannot be modified within the workflow.

## Use Case
- An outer workflow scope defines a variable's value.
- Subscopes should not be able to change the variable's value.
- Similar to readonly fields in C# classes where variables are initialized and then set to readonly.

## Benefits
- Enhances data integrity within workflows.
- Prevents accidental modifications of critical variables.
- Aligns with best practices in programming for managing state and immutability.