Introduce option to customize generation (only services, only clients, or both)

[darronschall]

Introduce an option, something like generate=<service|client|both>, to configure what code the plugin should generate.

• generate=service - only generate the ::Twirp::Service subclass.
• generate=client - only generate the ::Twirp::Client subclass.
• generate=both - generate both. Default option for backwards compatibility (and current behavior).

For generate=client, we don't want to use the client_for DSL because we do not have a service class reference to pass in (because one was not generated). Instead, we'll need to configure the client "manually".