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

[parthea]

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 without the use of interceptors:

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

[vchudnov-g]

This sounds straightforward to implement.

[noahdietz]

Does it make sense/seem possible to implement as part of the Sequence service? It is designed to test retry behavior so it already supports this requirement. The missing part would be ensuring/echoing back the request_id to enable the generator to assert on its generated client's behavior.