Add rpc with `google.cloud.operation_polling_method` to showcase protos

[parthea]

Commit googleapis/googleapis@e495ff5 had a failed build related to a gap in testing in gapic-generator-python for the annotation google.cloud.operation_polling_method . See tests/fragments/test_compute_operation.proto which was added in googleapis/gapic-generator-python#2014 to fix the testing gap.

This issue is to track adding google.cloud.operation_polling_method to gapic-showcase and https://google.aip.dev/

[noahdietz]

I am a little hesitant to put diregapic stuff in showcase b.c it isn't a diregapic API. When we start mixing things we may start crossing wires and breaking "normal" stuff.

In Go we have a "generation integration test" for Compute where we generate the Compute client and compile it to ensure it is valid. Similar in PHP, we use a generation integration test and run the unit tests.

Could you do something similar in Python?