[BUG] string(bool) overload is missing

[maorwayn]

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name: Bug report
about: Create a report to help us improve

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Describe the bug
When trying to run string(bool) in cel-java a CelValidationException for overload not found.

To Reproduce
Check which components this affects:

• parser
• checker
• runtime

Sample expression and input that reproduces the issue:

string(true)

Expected behavior
CEL should compile and run resulting a string.

[maorwayn]

I've created a PR with a fix: #727