Remove usage of spi Tuple classes under google-cloud-examples

[garrettjonesgoogle]

• https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-examples/src/main/java/com/google/cloud/examples/bigquery/BigQueryExample.java uses com.google.cloud.bigquery.spi.v2.BigQueryRpc.Tuple
• https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-examples/src/main/java/com/google/cloud/examples/storage/StorageExample.java uses com.google.cloud.storage.spi.v1.StorageRpc.Tuple

These tuple definitions are identical; maybe we should just make a Tuple class under core for everything to share.

[garrettjonesgoogle]

I'm marking this as "docs" because it is example code.

[garrettjonesgoogle]

Completed in #1992