Re-add generators objects

[marcospereira]

Power users can create instances of the case classes with powerApis = true, so no need to add another object here.

This keeps things consistent with how the generators are used in build.sbt.

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[raboof]

OK TO TEST

[raboof]

Isn't it a smell when this generator is used without specifying whether we want to generate power API's or not?

I'm OK with this change if it's useful for play-grpc though.

Eventually we'd want to remove this 'all-or-nothing' parameter from the generator, and instead specify it on a per-service (or even per-method) level, but that is future work (#451)