Commit 301d69d introduces a PluggableUSBModule::getProductVersion() method declaration, but it is not defined in the base class, resulting in a linker error when derived classes don't override it.

I believe this method should be pure virtual:

virtual uint8_t getProductVersion() = 0;

Is there a guideline for which value third-party libraries should use? Should they just set the next available bit (16)? Is there an upper bound?

From the name of the function alone, it was not really clear to me how the function is intended to be used. The links in the commit message explain the rationale behind it, but it might be useful to have a comment or docstring in the PluggableUSBModule class documenting how and why to use it when defining a module.