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

Commit https://github.com/googleapis/googleapis/commit/e495ff587351369637ecee17bfd260d2e76a41f7 had a failed [build](https://fusion2.corp.google.com/ci/kokoro/prod:actools%2Fgoogleapis%2Fstable%2Fcontinuous/activity/6c315bba-c325-42ac-819d-73ba4f89f31d/log) 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 https://github.com/googleapis/gapic-generator-python/pull/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/