[BUG] Start signing com.google.prefab:cli:2.0.0 artifacts

com.google.prefab:cli:2.0.0 artifacts in gmaven are currently not signed (missing .asc artifacts), which makes it not possible to verify if prefab was actually build by Google or not. A large number of libraries and build tools in maven already sign their artifacts, it would be great to see prefab do the same to allow signature validation when using it.