code: private CompletableFuture<Void> modifyColumns(ModifyTableBuilder modifications) {
      return toCompletableFuture(bigtableTableAdminClient
          .modifyFamiliesAsync(modifications.build()))
          .thenApply(r -> null);
    }
docs: <p>modifyColumns.</p>
  
  @param modifications a {@link ModifyTableBuilder} object.