Class Value (2.42.0-rc)

For now, this is a minimal class to unblock further development of the Bigtable GoogleSQL feature. It is conceptually similar to a std::any except the only allowed types will be those supported by Bigtable, and a "null" value (similar to a std::any without a value) still has an associated type. The supported types are shown in the following table along with how they map to the Bigtable types (https://cloud.google.com/bigtable/docs/data-types):

Callers may create instances by passing any of the supported values (shown in the table above) to the constructor. "Null" values are created using the MakeNullValue<T>() factory function or by passing an empty absl::optional<T> to the Value constructor.