Make it possible to specify visibility modifiers for generated classes/methods

[itegulov]

Motivation

There are two main use cases that I can see for this feature:

1. A library author that uses pbandk might want not want to expose auto-generated code to the end-users. For this, the author wants all classes generated by pbandk to have internal visibility modifier.
2. A library author that uses pbandk and employs explicit API mode who wants to expose proto classes to the end-users. For this, the author wants all classes generated by pbandk to have public visibility modifier.

Implementation

We are particularly interested in the second use-case and have implemented a very straightforward way to add visibility modifiers to all generated classes: input-output-hk@5577e0a. But, as with #173, we might want to explore a more flexible way via custom proto3 options.

[itegulov]

CC @vlad107

[garyp]

Thanks for detailing the use cases @itegulov! That was very helpful. I ran into the second use case when trying to enable explicit API mode for pbandk itself (for #134). I went ahead and added a visibility parameter to protoc-gen-pbandk, similar to what you did in https://github.com/input-output-hk/pbandk. Unlike in your version, I decided to default to explicit public visibility if the parameter is not provided. That reduces the number of variations of generated code that we have to worry about. I'll put up a pull request for this soon.

I agree that a more flexible way via proto3 custom options could be useful for some situations as well, so that you can change the Kotlin visibility per-file, per-message/enum, or even per-field. But even if we supported configuring the visibility via custom options, I think we'd still want to keep the protoc-gen-pbandk parameter since there are many use cases where it's preferable to configure the behavior of the code generator without modifying the actual .proto files.