Adapt constant folding to ignore functions returning Unknown

[zeitgeist87]

Some functions are intended to have side effects and therefore should not be folded even if all parameters are constants. Currently the constant folding optimizer throws an error if such a function does not provide an implementation. This change allows functions with side effects to return Unknown.

[TristonianJones]

/gcbrun

[TristonianJones]

To keep things simple, the constant folding optimizer should validate that all functions / overloads referenced in the ast.Expr have bindings before attempting to fold.

[TristonianJones]

In the side-effect case, we can propose marking a function as volatile or late-bound also skip trying to fold these functions.

cel.Function("withSideEffect", 
    cel.Overload("withSideEffect_int_int", []*cel.Type{cel.IntType, cel.IntType}, cel.IntType),
    cel.LateFunctionBinding())

This marker will let users specify that the function definition will be provided later on the cel.Activation.

// validates that the function has a late binding flag, and packages up context needed to resolve
// the function within the Activation by the interpreter.
lateFn, err := env.LateBoundFunction("withSideEffect", func(rhs, lhs ref.Val) ref.Val {
   // do something interesting
})
input, err := cel.NewActivation(data)
input.AddFunctionBinding(lateFn)

I'd like to do this to support late-bound functions inside of cel-go, so perhaps we can work toward the late-bound function support and solve two issues at once?