Feature request checklist

• There are no issues that match the desired change
• The change is large enough it can't be addressed with a simple Pull Request
• If this is a bug, please file a Bug Report.

Change
Currently, there is no option to remove built-in CEL functions when using cel.NewEnv. Also, there is no option to include standard built-in CEL functions when using cel.NewCustomEnv. The proposed change is to add an option to cel.NewEnv for excluding built-in functions or an option to cel.NewCustomEnv for including standard CEL functions.