chore(tests): add streaming retry showcase tests

[daniel-sanche]

This PR adds tests for api-core streaming retries in the gapic showcase system tests, using Streaming Sequences

There is currently one expected failing test: when testing a stream that sends partial data before raising an exception, the gprc rest client doesn't yield any of the partial data, only the exception. I'm not sure if this is caused by a bug in the showcase server, a bug in the rest client, or expected behavior. Let me know if you have any information

Other tests will pass after streaming retries branch is merged

[vchudnov-g]

Given the failure in googleapis/gapic-showcase#1377, we should ideally also do at least one manual check that the REST client's code handling server-streaming responses does indeed handle real server errors well.

[parthea]

Marking as draft as presubmits are failing

[parthea]

Closing as stale. Please re-open if this is still being worked on

[daniel-sanche]

This isn't something I'm currently working on, but I did put some work into this at one point, and I'd hate for it to get completely lost

The tests are failing because of a bug they turned up in the rest streaming code. I think originally we wanted to fix that so these tests would be green before merging, but the bug fix seems to have stalled. Maybe we should add a comment and skip those tests for now? Let me know how you want to proceed with this