Add @generated annotation to generated classes

Created by: zhaakhi

The Generated annotation can be used to mark generated Java code. https://docs.oracle.com/javase/8/docs/api/javax/annotation/Generated.html

It is used e.g. by error-prone to suppress warnings for generated code, see https://github.com/google/error-prone/issues/463

Behaviour of related tools: The Thrift compiler annotates with e.g. @javax.annotation.Generated(value = "Autogenerated by Thrift Compiler (0.10.0)", date = "2017-05-03")

AutoValue uses @Generated("com.google.auto.value.processor.AutoValueProcessor")

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