Add showcase to golden files

[parthea]

See googleapis/gapic-generator-python#1991 which was an attempt to add showcase to golden files but is blocked on #16312. It would be helpful to have showcase in golden files so that we can preview the impact that generator changes have on the showcase client. As an example, this would address the issue in googleapis/gapic-generator-python#1999 (comment)

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.