message: [tf.data service] Make server_lib visible.
  This enables users to use server_lib to build tf.data servers.
diff: @@ -275,6 +275,9 @@ cc_library(
  name = "server_lib",
  srcs = ["server_lib.cc"],
  hdrs = ["server_lib.h"],
  + visibility = [
  + "//visibility:public",
  + ],
  deps = [
  ":credentials_factory",
  ":grpc_master_impl",
  