feat: refactor generator options -> config

[shollyman]

This PR is a a larger diff, but conceptually it's straightforward:

• rename the options struct to generatorConfig
• rename the opts field in the generator type (main driver for the code generator) to cfg.
• newOptionsFromParams() -> configFromRequest()
• consolidate some of the config in the generator into this new generatorConfig struct, namely the API service config and the gRPC service config
• handle loading the grpc and api service configs during option processing. Currently we do this later in the generator setup process, and end up carrying around these file paths unnecessarily.
• update references

The ongoing goal is to get the generator have a more structured application model, and less commingling of random config and runtime state fields all over the place. I would also like to get closer to a world where config is immutable once built, so that the separation between config and state gets clearer.