Default the value to `:both` for backwards compatibility. Constraint the value to one of `:service`, `:client`, or `:both` and add spec to cover validation.

Note the differentiation here in client generation based on whether or not a service was also generated. When we have a service class, the client class can simply use the `client_for` DSL. Otherwise, we have to build the client from the service descriptor proto.

The complex example exercises more scenarios and will serve us better moving forward. Given that it also demonstrates correct namespace handling of imported types, the "import_type_retention" example was redundant.

There's some repeated code between "when specifying the `skip-empty` option" and when specifying the `skip-empty` and `generate=both` options" that we could do a better job of not repeating ourselves...