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

[garyp]

Discussed in #174

^{Originally posted by itegulov November 2, 2021}

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.