Null value.

`ListValue` is a wrapper around a repeated field of values.

`Struct` represents a structured data value, consisting of fields which map to dynamically typed values.

`Value` represents a dynamically typed value which can be either null, a number, a string, a boolean, a recursive struct value, or a list of values.

`NullValue` is a singleton enumeration to represent the null value for the `Value` type union.