Add `fail_n_times` field to add ability to configure a request to fail x number of times before receiving a valid response

Proposal:
Add a `Showcase.Echo` RPC with a `int fail_n_times` field that initially fails n number of times before returning a valid response.

This will allow us to test the following requirement from [AIP-4235](https://google.aip.dev/client-libraries/4235) without the use of interceptors:

- `Client libraries must reuse automatically populated values for retries of the same request.`