Support for variables in Custom Value inputs

### Which Data Mapper Version are you using ?

v2

### Describe the Bug with repro steps

Currently, when users need to provide a Custom Value as input to a function (e.g., a static string, a numeric constant, or a reference value), they must enter it manually each time while authoring function.

This becomes problematic when the same value is used across multiple functions, because:

- If the value needs to be updated later, the user must manually edit every instance where it appears.
- This is error-prone and time-consuming, especially in large mapping scenarios with repetitive references.

Expected Behavior:

- Users should be able to define and reference variables for Custom Values instead of hardcoding static inputs multiple times.

### Screenshots or Videos

*No response*

### Version

5\.38.0

### Additional context

*No response*