builder configurator properties retained when not null

Author: sagnghos

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

@@ -744,6 +744,9 @@ protected SpannerOptions(Builder builder) {
     if (builder.mTLSContext != null) {
       channelConfigurator =
           channelBuilder -> {
+            if (builder.channelConfigurator != null) {
+              channelBuilder = builder.channelConfigurator.apply(channelBuilder);
+            }
             if (channelBuilder instanceof NettyChannelBuilder) {
               ((NettyChannelBuilder) channelBuilder).sslContext(builder.mTLSContext);
             }