Allow generating only async or sync code

[anuraaga]

Fixes #209

Thanks for the idea @jeffsawatzky - I also know codebases that are relatively new and can be async-only, etc, so being able to simplify the generated code looks like a good idea.

I didn't provide separate options for client vs server - I think the use case is codebases, so either you're mixed and should generate all of it or you're not.

The option name itself was challenging - we use mode=async mode=sync in our unit tests but I don't think mode is a good word for a public API. So I went with a tri-state bool on just the option name async.

This doesn't touch the import statements for now - in the medium future we will be replacing the plugin framework with something smarter than string replacement, which will also handle import optimization. Until then, I'd like to avoid adding spaghetti to the import statements - it should work great to have ruff automatically optimize the imports after generation.

[stefanvanburen]

first time looking closely at the option parsing, but: should we be failing here on unrecognized option keys/values? (see https://github.com/connectrpc/connect-go/blob/e299aa60ffcf91633e48fcb3dd594cbbb187ec7f/cmd/protoc-gen-connect-go/main.go#L779)