Support list and context values for string() function

[saig0]

Describe the bug
Currently, the string() function doesn't work for list or context values. Instead, it returns null.

To Reproduce
Steps to reproduce the behavior:

1. Evaluate the expression string([1,2,3])
2. Verify that the result is null

Expected behavior
The string() function returns the string representation of list and context values. (see related DMN TCK case).

string([1, 2, 3])
// "[1, 2, 3]"

string({x: 1})
// "{x: 1}"

Environment

• FEEL engine version: 1.16.0
• Affects:
  • Camunda Automation Platform 7: [7.x]
  • Zeebe broker: [0.x]

[saig0]

Related OMG issue:

• https://issues.omg.org/browse/DMN16-13

[saig0]

The issue was fixed by #717 in version 1.17.0 as a side effect. 🚀

We need to add a few test cases to verify the behavior of the string() function before closing the issue.