Allow specifying a custom @Generated annotation

[josephearl]

As noted in the docs, the usefulness of markGenerated is limited and does not exclude classes from e.g. Jacoco coverage because all of the current annotation options are only retained in the source.

Allow specifying a custom annotation class e.g. markGenerated.set("com.cast.Generated") so users can create a @Generated annotation that is retained in the class file or at runtime.