Option to Remove Built-in CEL Functions

[sapirpol]

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.

[tonyhb]

I'd really like the ability to set *decls.Functions on an Env directly. We used to be able to achieve setting stdlib functions (or a subset) by configuring functions.StandardOverloads() then passing these overloads into the Env directly.

[TristonianJones]

@tonyhb @sapirpol this is a good feature request. I just haven't have time to work on it. I'm more in favor of preserving the functional options, but making it very easy to subset by type and function name.