bigquery: soon standard SQL will become the default

[Jonathan Amsterdam]

In a forthcoming release of the cloud.google.com/go/bigquery package, we will change the default version of SQL from Legacy to Standard. Legacy SQL itself won't be going away any time soon; only the default will change.

To ease the transition, we have added UseLegacySQL options for queries and table creation. If you want to continue using Legacy SQL without having to worry about the switchover, add these options to your code now.

Of course, you can also use your favorite package management tool (like dep) to pin your client to a specific version. We now regularly add semver tags to the cloud.google.com/go repo.