Detect and report unused proto types

[alexander-yevsyukov]

When working on tests in Validation I've discovered a number of proto types under test/proto that were not used at all.
This did not do any harm, but made tests harder to understand.

We need to detect such types and report them during the build process.

The same may apply to main proto types, but their usage and suggestion for removal should be done with greater care so that we don't break existing protocols.

Ideally such a feature should be available as a part of Spine SDK tooling so that the users can be reminded of "dead" generated code.

[alexander-yevsyukov]

@armiol, FYI.